Semiconductor device having a high breakdown voltage for use in communication systems

ABSTRACT

A HEMT has an InAlAs layer ( 202 ), an InGaAs layer ( 203 ), a multiple δ-doped InAlAs layer ( 204 ) composed of n-type doped layers ( 204   a ) and undoped layers ( 204   b ) which are alternately stacked, an InP layer ( 205 ), a Schottky gate electrode ( 210 ), a source electrode ( 209   a ), and a drain electrode ( 209   b ) on an InP substrate ( 201 ). When a current flows in a region (channel region) of the InGaAs layer ( 203 ) adjacent the interface between the InGaAs layer ( 203 ) and the multiple δ-doped InAlAs layer ( 204 ), a breakdown voltage in the OFF state can be increased, while resistance to the movement of carriers passing through the multiple δ-doped InAlAs layer ( 204 ) as a carrier supplying layer is reduced.

TECHNICAL FIELD

The present invention relates to a semiconductor device functioning as asemiconductor power device having a high breakdown voltage to be used tohandle an RF signal and to equipment for a communication system usingthe semiconductor device.

BACKGROUND ART

In recent years, new semiconductor materials (including so-calledsemi-insulating materials) for implementing semiconductor devices whichhave special functions and are particularly excellent in specificcharacteristics such as an RF characteristic, a light-emittingcharacteristic, and a breakdown voltage characteristic have been undervigorous development. Of the semiconductor materials, e.g., asemiconductor containing an indium phosphide (InP) as a main componentthereof is expected to be applied to a next-generation RF device,high-temperature operating device, and the like because the electronmobility and the saturation velocity of electrons are higher than thoseof silicon (Si) which is a typical semiconductor material.

In general, a power device is a generic name for a device which convertsor controls high power and is termed a power diode, a power transistor,or the like. Exemplary applications of the power device include aterminal in a communication system and a transistor disposed at a basestation. Examples of the transistor include a HEMT (High ElectronMobility Transistor) and a bipolar transistor. The applications of thepower device is expected to be widened in the future.

A typical modular structure used for such applications is obtained byconnecting a plurality of semiconductor chips each having a power deviceembedded therein with wires in accordance with a use or an object andplacing the connected semiconductor chips in a single package. Forexample, a desired circuit is constructed with semiconductor chips andwires by forming the wires on a substrate such that a circuit suitablefor the use is constructed and mounting the individual semiconductorchips on the substrate. A description will be given herein below to atransmitting/receiving circuit at a radio base station which uses aSchottky diode and a MESFET.

FIG. 27 is a block circuit diagram showing an internal structure of aconventional base station (base station in a mobile communicationsystem) disclosed in a document (Daisuke Ueda et al., “Radio-Frequencyand Optical Semiconductor Devices Exploring New Age of DataCommunication, IEICE, Dec. 1, 1999, p.124). As shown in the drawing, thecircuit comprises an antenna main body, a switch, a received-signalamplifier, an amplified-signal transmitter, a radiotransmitter/receiver, a baseband signal processor, an interface unit, anexchange controller, a controller, and a power supply portion. Thereceived-signal amplifier is composed of two filters and two low-noiseamplifiers (LNA) disposed in series. A mixer for mixing an output from alocal amplifier with an output from an RF emitter to generate an RFsignal is disposed at the radio transmitter/receiver. A powerdividing/synthesizing circuit having a driver amplifier, a filter, amiddle amplifier, and a main amplifier disposed therein is disposed atthe amplified-signal transmitter. There are further provided a basebandsignal processor for processing an audio signal, an interface unit, andan exchange controller connected to a network.

At the conventional base station, the main amplifier is so configured asto perform impedance matching by disposing an input matching circuit anda field-effect transistor (MESFET or HEMT) formed by using a GaAssubstrate, while disposing a capacitor, an inductor, and a resistorelement on each of the input side and output side.

A MOSFET formed on a silicon substrate, a diode, a capacitor, a resistorelement, and the like are disposed in the controller, the basebandsignal processor, the interface unit, and the exchange controller. Suchparts as a capacitor and an inductor which occupy a particularly largearea are formed as independent chips.

FIG. 28 schematically shows a structure of a conventional HEMT using anindium phosphide (InP) substrate. As shown in the drawing, an undopedInAlAs layer 502 with a thickness of about 200 nm, an undoped InGaAslayer 503 with a thickness of about 15 nm, an n-InAlAs layer 504 dopedwith silicon (Si) to serve as a carrier supplying layer with a thicknessof about 10 nm, an InP layer 505 serving as an etching stopping layerwith a thickness of about 5 nm, an n-InAlAs layer 506 doped with silicon(Si) and having a thickness of about 3 nm, an n⁺-InAlAs layer 507 dopedwith silicon (Si) as an n-type impurity at a high concentration andhaving a thickness of about 200 nm, and an n⁺-InGaAs layer 508 dopedwith silicon (Si) as an n-type impurity at a high concentration andhaving a thickness of about 15 nm are stacked successively on asemi-insulating InP substrate 501 doped with iron (Fe) at a highconcentration and having a thickness of about 100 μm. There are furtherprovided ohmic source/drain electrodes 509 a and 509 b each composed ofa TiAu film and provided in mutually spaced relation on the n⁺-InGaAslayer 508, a Schottky gate electrode 510 composed of WSi and penetratingrespective parts of the n-InAlAs layer 506, the n⁺-InAlAs layer 507, andthe n⁺-InGaAs layer 508 to be in contact with the InP layer 505, and aninsulating layer 511 composed of a SiO₂/SiN_(x) film for providing adielectric isolation between the Schottky electrode 510 and the ohmicsource/drain electrodes 509 a and 509 b.

In the transistor, if a voltage is applied between the source and drainelectrodes 509 a and 509 b, a current flows between the source and thedrain. If a voltage is applied between the Schottky gate electrode 510and the ohmic source electrode such that the Schottky gate electrode 510has a higher voltage (reverse voltage), the source/drain current ismodulated in accordance with the voltage applied to the Schottky gateelectrode 510 so that a switching operation is performed.

Subject Matters to be Solved

However, the conventional semiconductor device and the equipment for acommunication system using the conventional semiconductor device havethe following problems.

The breakdown voltage of the conventional transistor when pinch-offoccurred between the source and the drain is greatly dependent on adoping concentration in the n-InAlAs layer 50 functioning as the carriersupplying layer. To increase the breakdown voltage in the pinch-offstate, the doping concentration in the n-InAlAs layer 504 should bereduced to a low value. When the doping concentration is reduced,however, the resistance of the n-InAlAs layer 504 increases so that theON-state resistance when the transistor is turned ON is increased. As aresult, power consumption is increased. Due to such a trade-off, it hasbeen difficult to implement a transistor which is high in breakdownvoltage and low in resistance. Such a problem is also encountered if thetransistor is a bipolar transistor.

At the conventional base station, signal amplifying elements which arethe most important parts of the transmitting/receiving circuit aregenerally formed by using the GaAs substrate. However, the power is lowbecause of the trade-off between a high breakdown voltage and a lowresistance that has not been overcome so that a large number of elementsare required and it is difficult to scale down the device. To maintainthe base station, therefore, a cooling device having a high coolingability and high running cost are necessary. In the case of applying thetransmitting/receiving circuit to a terminal, there is a limit to thescaling down of the circuit.

In addition, power amplifiers which are the most important parts of thetransmitting/receiving circuit and the like are provided by disposing alarge number of signal amplifying elements at a portion at which aparticularly high power should be amplified. As the frequency of the RFsignal is higher, however, reflected waves from the MESFETs exertmultiple effects so that it is difficult to achieve impedance matching.This causes the disadvantage of increased labor required by trimming forimpedance adjustment.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide asemiconductor device which simultaneously achieves a high breakdownvoltage and a low resistance as described above, particularly asemiconductor device excellent in RF characteristic and equipment for acommunication system using the semiconductor device.

A semiconductor device according to the present invention comprises: atleast one first active region disposed on a substrate and composed of afirst semiconductor; and at least one second active region disposed incontact with the first active region and composed of a secondsemiconductor having a band gap different from a band gap of the firstsemiconductor such that a band discontinuity occurs between the firstand second active regions, the second active region being composed of:at least one first semiconductor layer which allows passage of carrierstherethrough; and at least one second semiconductor layer containing animpurity for carriers at a concentration higher than in the firstsemiconductor layer and smaller in film thickness than the firstsemiconductor layer such that carriers spread out to the firstsemiconductor layer under a quantum effect, the first and secondsemiconductor layers being disposed in contact with each other.

In the arrangement, the carriers in the second semiconductor layer ofthe second active region spread out extensively to the firstsemiconductor layer so that the carriers are distributed in the entiresecond active region. Since the impurity concentration is low in thefirst semiconductor layer during the operation of the semiconductordevice, scattering by impurity ions in the first semiconductor layer isreduced.

If a HEMT is used as the semiconductor device, therefore, a lowresistance is achieved in the second active region so that the carriersflow at a high speed. This provides a large current of carriers passingthrough the first active region. Moreover, the whole second activeregion is depleted in the OFF state irrespective of the mean impurityconcentration in the second active region which is relatively high sothat the carriers no more exist in the second active region.Consequently, the breakdown voltage is defined by the firstsemiconductor layer which is low in impurity concentration so that ahigh breakdown voltage is obtained in the entire second active region.Since the trade-off between a high breakdown voltage and a lowresistance is eased in the second active region, the power representedby the product of the voltage and current of the semiconductor devicecan be increased by using the effect. Even if the operating voltage isreduced, a sufficient driving power is obtainable so that thesemiconductor device is used also as a low-power-consumption device.

The first semiconductor layer includes a plurality of firstsemiconductor layers and the second semiconductor layer includes aplurality of second semiconductor layers, the first semiconductor layersand the second semiconductor layers being arranged in stacked relation.The arrangement provides a higher breakdown voltage and a lowerresistance more positively.

The substrate is composed of InP and each of the first and second activeregions is composed of any one material selected from the groupconsisting of InP, InGaAs, InAlAs, GaN, InGaP, and InGaSb. Thearrangement provides a semiconductor device suited for handling an RFsignal on the millimeter-wave level.

The first and second semiconductor layers in the second active regionare preferably composed of a common material.

The second semiconductor is composed of a material having a band gaplarger than a band gap of a material composing the first semiconductor,a portion of the first active region adjacent an interface between thefirst and second active regions serves as a channel layer, and thesecond active region functions as a carrier supplying layer, whereby ahigh-performance HEMT using a heterobarrier formed between the first andsecond active regions is obtained.

The second active region includes two second active regions providedabove and below the first active region. The arrangement provides adouble-channel HEMT suitable for a power device.

The first active region includes a plurality of first active regions andthe second active region includes a plurality of second active regions,the first active regions and the second active being arranged in stackedrelation. The arrangement provides a multi-channel HEMT more suitablefor a power device.

Equipment for a communication system according to the present inventionis equipment for a communication system handling an RF signal, theequipment being disposed in the communication system and having anactive element formed by using a semiconductor, the active elementcomprising: at least one first active region disposed on a substrate andcomposed of a first semiconductor; and at least one second active regiondisposed in contact with the first active region and composed of asecond semiconductor having a band gap different from a band gap of thefirst semiconductor such that a band discontinuity occurs between thefirst and second active regions, the second active region being composedof: at least one first semiconductor layer which allows passage ofcarriers therethrough; and at least one second semiconductor layercontaining an impurity for carriers at a concentration higher than inthe first semiconductor layer and smaller in film thickness than thefirst semiconductor layer such that carriers spread out to the firstsemiconductor layer under a quantum effect, the first and secondsemiconductor layers being disposed in contact to each other.

The arrangement provides the equipment for a communication systemsmaller in size by using the active element capable of increasing anamount of current by increasing a breakdown voltage and reducing aresistance, while taking advantage of the various characteristics of aheterojunction semiconductor device, and reduces placement cost, runningcost, and the like for facilities.

The active element is disposed at a transmitter. The arrangement allowsfull use of the active element capable of providing a high power asdescribed above at the portion of the communication system whichrequires the highest power.

The active element may be disposed at a receiver.

The active element is disposed at a mobile data terminal. Thearrangement allows full use of the active element small in size andsuited for reducing power consumption.

The active element is disposed at a base station. The arrangement allowsthe active element to be used as an RF device and as a power device.

The equipment for a communication system is a transmitting/receivingmodule constructed attachably to and removably from an object undercontrol. The arrangement allows control of the object by using newsoftware without changing the content of the object under control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an example of the overallconcept of a communication system using a millimeter wave according toan embodiment of the present invention;

FIG. 2 is a block diagram schematically showing a structure of acommunication system in the embodiment;

FIG. 3 is a block circuit diagram showing in detail an internalstructure of a base station in a communication system in the embodiment;

FIG. 4 is an electric circuit diagram showing an exemplary structure ofa main amplifier disposed in an amplified-signal transmitter or areceived-signal amplifier shown in FIG. 2;

FIG. 5 schematically shows an example of an in-home broadband radiocommunication system;

FIG. 6 is a block diagram for illustrating a method for conducting radiocommunications among an ONU/server, a home electrical appliance, and anin-home mobile data terminal by using communication cards.

FIG. 7 is a cross-sectional view of a semiconductor device obtained byintegrating a Schottky diode, a HEMT, a MESFET, a capacitor, and aninductor in an InP substrate in the embodiment;

FIG. 8A diagrammatically shows the relationship between theconcentration profile of nitrogen and the distribution of carriers alongthe depth of a multiple δ-doped InGaAs layer in the embodiment and FIG.8B is a partial band diagram showing the configuration of a conductionband edge;

FIG. 9 shows the distribution of a dopant concentration along the depthof the multiple δ-doped layer in a SiC layer;

FIG. 10A shows the result of simulating a band structure at a conductionband edge in a sample A having a multilayer portion composed of δ-dopedlayers and undoped layers and FIG. 10B shows the result of simulatingthe distribution of carrier concentration;

FIG. 11A shows the result of simulating a band structure at a conductionband edge in a sample B having a multilayer portion composed of δ-dopedlayers and undoped layers and FIG. 11B shows the result of simulatingthe distribution of carrier concentration;

FIG. 12A1 to FIG. 12C3 are energy band diagrams each showing changes inthe configuration of a conduction band edge caused by changes in bias inthe Schottky diode according to the present embodiment and in aconventional Schottky diode;

FIG. 13 shows the result of measuring the dependence of the relationshipbetween drain current and drain voltage on gate voltage (I-Vcharacteristic) in a MESFET having a multiple δ-doped layer;

FIG. 14A to FIG. 14C are cross-sectional views illustrating processsteps of fabricating the semiconductor device according to theembodiment from the formation of a multiple δ-doped InGaAs layer, amultiple δ-doped InAlAs layer, and the like to the formation of anisolation regions;

FIG. 15A and FIG. 15B are cross-sectional views illustrating the processsteps of fabricating the semiconductor device according to the presentembodiment from the formation of the respective electrodes of individualelements to the formation of a conductor film;

FIG. 16A and FIG. 16B are cross-sectional views illustrating the processsteps of fabricating the semiconductor device according to theembodiment from the formation of the upper electrodes of capacitors tothe formation of contact holes connecting to the respective conductorportions of the elements;

FIG. 17 is a cross-sectional view of a HEMT in a first example of theembodiment of the present invention;

FIG. 18A and FIG. 18B are energy band diagrams schematically showing therespective states of a band during no application of a bias to aheterojunction portion in the HEMT of the first example and during theapplication thereof;

FIG. 19 is a cross-sectional view of a HEMT in a second example of theembodiment of the present invention;

FIG. 20A and FIG. 20B are energy band diagrams schematically showing therespective states of a band during no application of a bias to aheterojunction portion in the HEMT of the second example and during theapplication thereof;

FIG. 21 schematically shows an example of a mobile phone terminal(mobile station) in the communication system shown in FIG. 2;

FIG. 22 is an electric circuit diagram showing an exemplary circuitconfiguration of a mixer shown in FIG. 3 or 21;

FIG. 23 is an electric circuit diagram showing an example of ahigh-output switch circuit containing an SPDT switch shown in FIG. 21 ora high-output switch circuit disposed in the switch shown in FIG. 3;

FIG. 24 shows another exemplary structure (first variation) of the mainamplifier shown in FIG. 4 according to the embodiment;

FIG. 25 shows still another exemplary structure (second variation) ofthe main amplifier shown in FIG. 4 according to the embodiment;

FIG. 26 is a block circuit diagram schematically showing a structure ofa base station in a third variation in which two main amplifiers aredisposed in parallel;

FIG. 27 is a block circuit diagram showing an internal structure of aconventional base station (base station in a communication system);

FIG. 28 schematically shows a structure of a conventional HEMT using anInP substrate; and

FIG. 29 shows the characteristics of a power amplifier determined asspeculations for a mobile data terminal.

BEST MODES FOR CARRYING OUT THE INVENTION

Basic Structure of Communication System

FIG. 1 is a perspective view showing an example of the overall conceptof a communication system (network system) using a millimeter waveaccording to an embodiment of the present invention. As shown in thedrawing, a base station is provided on a tip of each of a large numberof optical fibers branched from a trunk optical fiber line (Trunk LineO-Fiber). There is also formed a radio communication network forconducting communication from each of the base stations to each of homes(or offices) by using a millimeter wave. At a radio-wave terminal(mobile station) at each of homes or offices, there can be performed thereception of various media supplied from the base station to equipmentat the home or office, Internet communication, communication betweenmobile stations, and the like. Since a millimeter wave has a wavelengthclose to that of light, it is susceptible to interference from anobject. Accordingly, data is transmitted and received to and from thebase station by optical communication via an optical fiber network, aconversion is made between an optical signal and an electric signal atthe base station, and a wireless access using the millimeter wave isenabled between each of homes or offices and the base station. At partof the system, a wireless access is enabled via an antenna between thebase station connected directly to the trunk optical fiber line and amobile data terminal or a terminal at an office.

FIG. 2 is a block diagram schematically showing a structure of thecommunication system between the base station and the radio-waveterminal at each of homes or offices shown in FIG. 1. As shown in FIG.2, the communication system of the embodiment comprises: a large numberof base stations 101 connected to each other via an optical fibernetwork 100; and radio-wave terminals 102 for communicating with eachother via the individual base stations 101. Each of the base stations101 comprises: an antenna device 111 for receiving and transmitting aradio wave; a received-signal amplifier 112 having the function ofamplifying a radio signal received at the antenna device 111 and thelike; an amplified-signal transmitter 113 for sending an amplified RFsignal to the antenna device 111; a radio receiver/transmitter 114connected to the received-signal amplifier 112 and to theamplified-signal transmitter 113; a controller 115 for controlling theoperation of each of the devices; and a wire connector 116 forconnecting signals between the base station 101 and the optical fibernetwork 100. Each of the radio-wave terminals 102 comprises: a switch121 for receiving and transmitting a radio wave; a received-signalamplifier 122 having the function of amplifying a radio signal receivedat the switch 121 and the like; an amplified-signal transmitter 123 forsending an amplified RF signal to the switch 121; and a controller 125for controlling the operation of each of the devices.

FIG. 3 is a block circuit diagram showing in detail an internalstructure of the base station 101. As shown in the drawing, the antennadevice 111 is composed of: an antenna main body 111 a; and a switch 111b for switching the antenna main body 111 a between transmission andreception. The received-signal amplifier 112 is composed of two filters131 and two low-noise amplifiers (LNA) 132 disposed in series. A mixer134 for mixing an output from a local amplifier with an output from anRF transmitter to generate an RF signal is disposed in the radiotransmitter/receiver 114. A driver amplifier 135, a filter 136, a middleamplifier 137, and a main amplifier 138 are disposed in theamplified-signal transmitter 113. The wire connector 116 is composed ofa baseband signal processor 117 for processing an audio signal, aninterface unit 118, and an exchange controller 119 connected to theoptical fiber network 100. The interface unit 118 is provided with asignal converter for making a conversion between an optical signal andan electric signal, though it is not depicted.

FIG. 4 is an electric circuit diagram showing an exemplary structure ofthe main amplifier 138 disposed in the amplified-signal transmitter 113shown in FIG. 3. As shown in FIG. 3, the main amplifier 138 is composedof a HEMT disposed therein and having a gate for receiving an inputsignal Pin and a drain for outputting an output signal Pout. A gate biasVg is applied to the gate of the HEMT via a resistor Rg and a powersupply voltage Vd is applied to the drain of the HEMT via a chokeinductor, while the source of the HEMT is connected to the ground. Aninput-side circuit is provided with an input terminal Tin for supplyingthe input signal Pin to the HEMT via an input-side circuit, a signalsupply for supplying electric power to the input terminal Tin via asignal supply resistor R_(S), and capacitors C1 and Cin and a microstripline each composing an input-side impedance matching circuit. Anoutput-side circuit is provided with an output terminal Tout for sendingan output signal to the outside via the output-side circuit, capacitorsC2 and Cout and a microstrip line each composing an output-sideimpedance matching circuit, and a load resistor R_(L) interposed betweenthe output terminal Tout and the ground. If a bipolar transistor is usedin place of the HEMT, a diode indicated by the broken line may bedisposed between the emitter of the bipolar transistor and the ground.

A power amplifier used for mobile communication is required of suchcharacteristics as high efficiency and low distortion. In a typical RFpower device, the relationship between efficiency and distortion is atrade-off. In the power amplifier it is important to increaseefficiency, while achieving low distortion. As shown in the drawing, thecapacitance value of the capacitor Cin connected in parallel to themicrostrip line and the length Lin of the microstrip line are adjustedin the input-side impedance matching circuit such that a reflectioncoefficient when the MESFET is viewed from the input terminal Pin isminimized. Each of the capacitors C1 and C2 is for blocking a current,which forms a sufficiently low impedance in an RF region. The resistorRg for supplying the gate bias has been set to a value higher than agate input impedance such that RF power does not leak. The inductance ofthe choke inductor L for supplying a drain bias, the capacitances of thecapacitors C1 and C2, and the resistance of the resistor Rg do notaffect an impedance in the RF region.

FIG. 5 schematically shows an example of an in-home broadband radiocommunication system. The system comprises: an ONU (Optical NetworkUnit) in which an opto-electric converter for converting, to an electricsignal, an optical signal transmitted from an optical fiber line andconverting, to an optical signal, an electric signal transmitted from ahome to the optical fiber line, a demultiplexer, and the like aredisposed; and a server connected to the ONU. In the in-home broadbandcommunication system, there are provided home electrical appliances suchas a personal computer, a printer, an in-home mobile data terminal(PDA), a TV set, a CD player, a refrigerator, an air conditioner, and amicrowave oven which are connected to the server by radiocommunications. A communication card which is a transmitting/receivingmodule having a broadband millimeter-wave integrated circuit(millimeter-wave LSI), a CPU, and a memory is constructed attachably toand removably from each of the server, the home electrical appliances,and the like. The broadband millimeter-wave integrated circuit(millimeter-wave LSI) has a large number of devices (such as atransistor, a diode, a capacitor, and an inductor) for processing abroadband millimeter-wave signal mounted thereon and an additionalantenna for reception and transmission provided therein. Instead ofbeing formed as the communication card, the transmitting/receivingmodule may also be incorporated previously in each of the appliances. Ifthe communication card is used, it is constructed such that a userselectively controls a home electrical appliance by using thecommunication card or the like or by using a button, a switch, or thelike provided additionally on the home electrical appliance.

FIG. 6 is a block diagram for illustrating a method for conducting radiocommunications among an ONU/server, a home electrical appliance, and anin-home mobile data terminal by using communication cards. As shown inthe drawing, if the communication card is inserted in each of theappliances, the operation of the appliance is brought into acontrollable state by a network computer. The control can also beeffected by using software from Internet without using a personalcomputer. If the communication card is replaced with another, the homeelectrical appliance can also be controlled by using a new application.It is also possible to construct an in-home radio LAN between electricappliances by using software supplied from the network. Since closeconnection can be provided between the CPU and the memory in thecommunication card, a load on each of the home electrical appliances canbe reduced.

Thus, the communication system allows control of the home electricalappliances and the like by using the communication cards having thecharacteristics of mobility, reduced size, large capacity, andflexibility. The system is also allowed to function as a home securitysystem indicating a visitor, a fire, and the like or as an ITS(Intelligent Transport System) for a car. In that case, thetransmitting/receiving module (communication card) is properly attachedto the car or incorporated previously in the control device of the car.

It is to be noted that a circuit having the same function as areceived-signal amplifier, an amplified-signal transmitter, and acontrol unit similar to those of the base station or the mobile dataterminal shown in FIG. 2 is also disposed in the communication card(transmitting/receiving module). The specific structures thereof arebasically the same as those shown in FIGS. 3 and 4.

The broadband millimeter-wave integrated circuit in the communicationcard (transmitting/receiving module) disposed in such an in-homebroadband radio communication system need not necessarily comprise amultiple δ-doped layer. If a conventional broadband millimeter-waveintegrated circuit having a GaAs-MESFET, a GaAs-HEMT, and the likedisposed therein is incorporated into the communication card or thetransmitting/receiving module and the home electrical appliances arecontrolled by using the communication card or the transmitting/receivingmodule, e.g., the effects of unprecedented simplicity and convenience(mobility, reduced size, and flexibility) can be achieved.

Instead of the communication card, a memory device having the samefunction may also be used.

Example of Semiconductor Integrated Circuit Device

A description will be given herein below to an example of asemiconductor integrated circuit device formed by integrating activeelements such as a transistor and a diode disposed in equipment for acommunication system such as a base station, a mobile data terminal(PDA), or a transmitting/receiving module (a module disposed in a homeelectrical appliance or a communication card) with passive elements suchas a capacitor and an inductor, which characterizes the presentinvention. Although the semiconductor integrated circuit devicedescribed herein below has a structure of an MMIC, the semiconductorintegrated circuit device according to the present invention is notlimited to a modulized device such as the MMIC.

FIG. 7 is a cross-sectional view of a semiconductor integrated circuitdevice (MMIC) formed by integrating a Schottky diode, a HEMT, a MESFET,a capacitor, and an inductor in an InP substrate in the embodiment ofthe present invention.

A semi-insulating InP substrate 10 doped with iron (Fe) at a highconcentration and having a thickness of about 100 μm is provided with:an undoped InAlAs layer 15 (in which ratios of components are, e.g.,In_(0.52)Al_(0.48)As) serving as a first active region with a thicknessof about 200 nm; a multiple δ-doped InGaAs layer 12 (in which ratios ofcomponents are, e.g., In_(0.53)Ga_(0.47)As) serving as a second activeregion with a thickness of about 70 nm; an undoped InAlAs layer 16 (inwhich ratios of components are, e.g., In_(0.52)Al_(0.48)As) with athickness of about 10 nm; an undoped InGaAs layer 17 (in which ratios ofcomponents are, e.g., In_(0.53)Ga_(0.47)As) with a thickness of about 10nm; a multiple δ-doped InAlAs layer 13 (in which ratios of componentsare, e.g., In_(0.52)Al_(0.48)As) with a thickness of about 65 nm; and anInP layer 18 serving as an etching stopping layer with a thickness ofabout 5 nm.

As shown in the lower part of FIG. 7 under magnification, the multipleδ-doped InGaAs layer 12 is composed of five n-type doped layers 12 aeach composed of an InGaAs single crystal (in which ratios of componentsare, e.g., In_(0.53)Ga_(0.47)As) containing Si (silicon) at a highconcentration (e.g., 1×10 ²⁰ atoms cm⁻³) and having a thickness of about1 nm and six undoped layers 12 b each composed of an InGaAs singlecrystal (in which ratios of components are, e.g., In_(0.53)Ga_(0.47)As)and having a thickness of about 10 nm, which are alternately stacked. Onthe other hand, the multiple δ-doped InAlAs layer 13 is composed of fiven-type doped layers 13 a each containing Si at a high concentration(e.g., 1×10²⁰ atoms cm⁻³) and having a thickness of about 1 nm and sixundoped layers 12 b each composed of an undoped InAlAs single crystal(in which ratios of components are, e.g., In_(0.52)Al_(0.48)As) andhaving a thickness of about 10 nm, which are alternately stacked. Eachof the n-type doped layers 12 a and 13 a is formed sufficiently thin toallow spreading movement of carriers to the undoped layers 12 b and 13 bunder a quantum effect. As will be described later, an impurityconcentration profile in each of the n-type doped layers has a generallyδ-functional configuration relative to the underlying undoped layer.Therefore, the present specification refers to the n-type doped layers12 a and 13 a as so-called δ-doped layers. The present specificationwill also refer to a structure consisting of a plurality of heavilydoped layers (δ-doped layers) and a plurality of lightly doped layers(undoped layers), which are alternately stacked, as a multiple δ-dopedlayer.

A Schottky diode (rectifying element) 20 and a MESFET (power amplifier)30 are provided on that portion of the InP substrate 10 at which themultiple δ-doped InGaAs layer 12 is exposed. An HEMT (power amplifier)40, a capacitor (capacitor element) 50, and an inductor (inducingelement) 60 are provided on that portion of the InP substrate 10 havingthe multiple δ-doped InAlAs layer 13 located in the uppermost positionthereof. In short, the HEMT composing the main amplifier 138 of theamplified-signal transmitter 113 shown in FIG. 4, the diode (the portionindicated by the broken line), the capacitor, the inductor, and theMESFET disposed in a circuit for amplifying an RF signal in thefrequency region lower than the millimeter-wave region are provided onthe single InP substrate.

It is unnecessary for the MESFET and the HEMT to be providedsimultaneously in the integrated circuit device. Either one of theMESFET and the HEMT may be provided appropriately. Normal MISFETs(particularly a PMISFET and an nMISFET) may also be provided.

The Schottky diode 20 comprises a Schottky electrode 21 composed ofTiPtAu in Schottky contact with the multiple δ-doped InGaAs layer 12, awithdrawn electrode layer 22 formed by implanting Si at a highconcentration (e.g., about 1×10¹⁸ atoms cm⁻³) into the multiple δ-dopedInGaAs layer 12, and an ohmic electrode 23 composed of a TiPtAu film inohmic contact with the withdrawn electrode layer 22.

The MESFET 30 comprises a Schottky gate electrode 32 composed of aTiPtAu film in Schottky contact with the undoped layer 12 b serving asthe uppermost layer of the multiple δ-doped InGaAs layer 12 andsource/drain electrodes 34 and 35 which are provided on the regions ofthe multiple δ-doped InGaAs layer 12 located on both sides of the gateelectrode 32 and in ohmic contact with the multiple δ-doped InGaAs layerportion 12. It is to be noted that Si at a high concentration has beenintroduced into the regions of the multiple δ-doped InGaAs layer 12 incontact with the source/drain electrodes 34 and 35.

The HEMT 40 comprises a Schottky gate electrode 42 composed of a TiPtAufilm in Schottky contact with the InP layer 18 and source/drainelectrodes 44 and 45 composed of a TiPtAu film in ohmic contact with therespective regions of the InP layer 18 located on both sides of the gateelectrode 42.

The capacitor 50 comprises an underlying insulating film 51 composed ofa SiN film provided on the InP layer 18, a lower electrode 52 composedof a platinum (Pt) film provided on the underlying insulating film 51, acapacitor insulating film 53 composed of a high dielectric film such asBST provided on the lower electrode 52, and an upper electrode 54composed of a platinum (Pt) film opposed to the lower electrode 52 withthe capacitor insulating film 53 interposed therebetween.

The inductor 60 comprises a dielectric film 61 composed of a SiN filmprovided on the InP layer 18 and a conductor film 62 composed of aspiral Cu film formed on the dielectric film 61. The conductor film 62has a width of about 9 μm and a thickness of about 4 μm. The spacingbetween the conductor films 62 is about 4 μm. However, since the InPsubstrate 10 has a high heat resistance and a high heat conductivity,the conductor film 62 can be scaled down depending on an amount ofcurrent into a miniaturized pattern, e.g., a configuration with a widthof about 1 to 2 μm and a spacing of about 1 to 2 μm.

An interlayer insulating film 70 composed of a silicon dioxide film isalso formed on the substrate. Wires (not shown) composed of an aluminumalloy film, a Cu alloy film, or the like are provided on the interlayerinsulating film 70. The elements 20, 30, 40, 50, and 60 have respectiveconductor portions connected to the wires via contacts 71 composed of analuminum alloy film buried in contact holes formed in the interlayerinsulating film 70, whereby circuits at individual base stations areconstructed as shown in FIG. 3. However, it is unnecessary for all thecircuits shown in FIG. 3 to be provided on a single InP substrate. Anyof the elements may also be provided on another substrate (siliconsubstrate). For example, although the amplified-signal transmitter andthe received-signal amplifier, each requiring a power element, areprovided on the InP substrate, the baseband processor which does notrequire a power element may also be provided on a silicon substrate.

In the present embodiment, principal devices in the equipment for acommunication system such as a base station, a mobile data terminal, ora transmitting/receiving module are mounted on a single InP substrate sothat a required circuit is scaled down, as shown in FIG. 7. Accordingly,the equipment for a communication system (e.g., equipment including thewhole circuit shown in FIG. 2) in the present embodiment can be scaleddown and the total thickness thereof is only on the order of the sum ofthe thickness of the InP substrate and the respective thicknesses of themultilayer film and the interlayer insulating film so that the wholeequipment for a communication system such as a base station, a mobiledata terminal, or a transmitting/receiving module has an extremely thinstructure. In other words, the size of the equipment for a communicationsystem such as a base station, a mobile data terminal, or atransmitting/receiving module itself can be reduced. In particular,integration is facilitated since the HEMT, the MESFET, the Schottkydiode, and the like can be provided on the single InP substrate byforming the Schottky diode into a lateral configuration, as shown inFIG. 7. A further size reduction is achievable by mounting even passiveelements such as an inductor and a capacitor on the common InPsubstrate.

It has been known that the use of an InGaAs layer formed on an InPsubstrate as an electron flow region provides a particularly highelectron mobility. By using the characteristic, a HEMT handling an RFsignal in a frequency region on the order of the millimeter-wave region(30 GHz to 60 GHz) can be obtained. This allows the formation of an MMICon which a power amplifier capable of amplifying a signal in thefrequency region of 30 GHz to 60 GHz is mounted.

The significant size reduction of the circuit provides high flexibilitywith which individual members are placed at a base station, a mobiledata terminal (mobile station), a transmitting/receiving module, or thelike.

As a result, there is provided a semiconductor device having thecharacteristics of high power and a high breakdown voltage and suitablefor use at a base station, a mobile data terminal (mobile station), atransmitting/receiving module, or the like in a communication system.Even if the semiconductor device is placed at a base station or in ahome electrical appliance, the size reduction of the circuit obviatesthe necessity to provide a cooling device having a particularly highcooling ability. This reduces placement cost for cooling facilities andrunning cost for power and the like.

By integrating a majority of elements in the base station, the mobiledata terminal (mobile station), or the transmitting/receiving moduleinto a common InP substrate, the labor of assembling parts can be savedand the fabrication cost for the semiconductor device can be reduced.Because the element having the multiple δ-doped layer composed of theδ-doped layers and the undoped layers which are arranged in stackedrelation increases the reliability of the device, a higher productionyield can also be expected so that a cost reduction due to the higherproduction is achieved.

If the semiconductor device is applied to equipment handling an RFsignal on the GHz order, in particular, the dielectric film 61 of theinductor 60 is preferably composed of a BCB (benzocyclobutene) film. TheBCB film is a film containing BCB in the structure thereof, which isobtained by dissolving a BCB-DVS monomer in a solvent, applying theresulting solution, and baking the applied solution. The BCB filmfeatures a relative dielectric constant as low as about 2.7 and easyformation of a film as thick as about 30 μm by a single step ofapplication. Since the tan δ of the BCB film is about 0.006 at 60 GHz,which is lower than that of SiO₂ by one order of magnitude, the BCB filmhas particularly excellent characteristics as the dielectric filmcomposing the inductor and the microstrip line.

Multiple δ-doped Layer

As stated previously, the semiconductor device according to the presentembodiment comprises the multiple δ-doped layer composed of the n-typedoped layers 12 a or 13 a and the undoped layers 12 b or 13 b which arealternately stacked. Such a structure composed of heavily doped layers(δ-doped layers) and lightly doped layers (undoped layers) which arealternately stacked is obtainable by using the crystal growing apparatusand the crystal growing method disclosed in the specifications anddrawings of Japanese Patent Applications 2000-58964, 2000-06210, and thelike, which will be described later. Specifically, an epitaxial growingmethod using in-situ doping is used by simultaneously effecting thesupply of a dopant gas using a pulse valve (termed pulse doping) and thesupply of a source gas. A description will be given herein below to thesignificance of the multiple δ-doped layer according to the presentinvention.

FIG. 8A diagrammatically shows the relationship between theconcentration profile of nitrogen as an n-type impurity and thedistribution of carriers along the depth of the multiple δ-doped layeraccording to the present embodiment (the multiple δ-doped InGaAs layer12 or the multiple δ-doped InAlAs layer 13). FIG. 8B is a partial banddiagram showing the configuration of a conduction band edge along thedepth of the multiple δ-doped InGaAs layer.

Since the thickness of each of the n-type doped layers 12 a and 13 a isas thin as about 10 nm as shown in FIG. 8A and FIG. 8B, a quantum levelresulting from a quantum effect occurs in each of the n-type dopedlayers 12 a and 13 a so that the wave function of an electron presentlocally in each of the n-type doped layers 12 a and 13 a expands to acertain degree. What results is a state of distribution in whichcarriers are present not only in the n-type doped layers 12 a and 13 abut also in each of the undoped layers 12 b and 13 b at a concentrationhigher than an original concentration, as indicated by the broken curvein the drawing. In the state in which the potential of the multipleδ-doped layer is increased and carriers flow, electrons are constantlysupplied to the n-type doped layers 12 a and 13 a and to the undopedlayers 12 b and 13 b. What results is the state of distribution in whichelectrons are present not only in the n-type doped layers 12 a and 13 abut also in each of the undoped layers 12 b and 13 b at a relativelyhigh concentration. Since electrons flow not only in the n-type dopedlayers 12 a and 13 a but also in the undoped layers 12 b and 13 b, theresistance of the multiple δ-doped layer is reduced. This reducesscattering by impurity ions in the undoped layers 12 b and 13 b andthereby achieves a particularly high electron mobility in each of theundoped layers 12 b and 13 b.

In the state in which the entire multiple δ-doped layer is depleted, onthe other hand, carriers no more exist in the undoped layers 12 b or 13b and in the n-type doped layers 12 a or 13 a so that a breakdownvoltage is defined by the undoped layers 12 b and 13 b each having alower impurity concentration and the whole multiple δ-doped layer (themultiple δ-doped InGaAs layer 12 or the multiple δ-doped InAlAs layer13) has a high breakdown voltage.

The foregoing fundamental effects are achievable not only when aplurality of δ-doped layers are present in the carrier flow region butalso when a single δ-doped layer is present therein. If at least oneδ-doped layer is present in the carrier flow region serving as adepletion layer when a voltage at which the device operates is applied,carriers spread out from the δ-doped layer to the adjacent undoped layer(lightly doped layer) so that the carriers flow in the region reached bythe carriers from the undoped layer, so that a low resistance isobtainable under the foregoing effect. If the device is in the OFFstate, on the other hand, the δ-doped layer is also depleted so that ahigh breakdown voltage is obtained. Thus, if at least one δ-doped layeris present in the carrier flow region when a voltage at which the deviceoperates (set ON voltage) is applied, both the low resistance and thehigh breakdown voltage can be achieved at the same time.

Each of the foregoing effects are similarly achievable even if holes,not electrons, are used as carriers.

As shown in FIG. 8B, a conduction band edge in the whole multipleδ-doped layer presents a configuration which connects a conduction bandedge in the n-type doped layer (δ-doped layer) 12 a or 13 a indicated bythe broken line to a conduction band edge in the undoped layer 12 b or13 b indicated by the broken line. Although it is normal to increase theimpurity concentration in the n-type doped layer 12 a or 13 a to a valueat which the conduction band edge therein is lower than the Fermi levelE_(f), the impurity concentration in the n-type doped layer 12 a neednot necessarily be increased to such a high value.

In the multiple δ-doped layer having p-type δ-doped layers also, therelationship between the Fermi level E_(f) and a valence band edge inthe δ-doped layer is represented by a configuration obtained byreplacing the conduction band edge with a valence band edge in FIG. 8Band vertically inverting the whole diagram.

By using the multiple δ-doped layer (the multiple δ-doped InGaAs layer12 or the multiple δ-doped InAlAs layer 13 in the present embodiment)having such structures as the carrier flow region, a high-performancedevice as will be shown in the following embodiment is obtainable. Asfor the function of the δ-doped layers and the undoped layers serving asthe carrier flow region in the multilayer δ-doped layers, it will bedescribed in the following embodiment.

Although the present embodiment has formed the δ-doped layer on theundoped layer, it is also possible to use an n-type lightly doped layeror a p-type lightly doped layer formed by opening the pulse valveinstead of the undoped layer.

Although the present embodiment has described the CVD method usinginduced heating as a method for growing a thin film on a substratematerial, it will easily be appreciated that the thin-film growingmethod according to the present invention is effective even when a thinfilm is grown on the base material under the effects achieved by any ofplasma CVD, photo assisted CVD, and electron assisted CVD.

The present invention is also applicable to a multilayer structurecomposed of lightly doped layers (including undoped layers) and heavilydoped layers thinner than the lightly doped layers to a degree whichallows carriers to spread out to the lightly doped layers under aquantum effect by using not only CVD but also another technique such assputtering, vapor deposition, or MBE.

Experimental Data

A description will be given herein below to the relationship between thethickness of the multiple δ-doped layer and the effects achieved therebybased on exemplary experiments disclosed in a PCT application(PCT/JP00/01855) by the present inventors, which are related to amultiple δ-doped layer in a SiC layer.

FIG. 9 shows the distribution of a dopant concentration along the depthof the multiple δ-doped layer in the SiC layer. As described above, theperiod (pulse width) during which a pulse valve is open during theformation of the n-type doped layer is adjusted to 102 μs and the periodduring which the pulse valve is closed (interval between pulses) isadjusted to 4 ms. The concentration profile of FIG. 9 was obtained as aresult of measurement performed by using a secondary ion massspectrometry (SIMS). In FIG. 9, the horizontal axis represents a depth(μm) from the uppermost surface of the substrate) and the vertical axisrepresents the concentration of nitrogen as the dopant. As shown in thedrawing, the concentration of nitrogen (N) in each of the n-type dopedlayers formed in accordance with this method is nearly uniform (about1×10¹⁰ ¹⁸ atoms cm⁻³) and the impurity concentration changes extremelysharply in each of the region in which the undoped layer shifts to then-type doped layer and the region in which the n-type doped layer shiftsto the undoped layer. In the present embodiment also, the multipleδ-doped layer having an impurity profile as shown in FIG. 8 can beformed easily by supplying a dopant gas containing Si as a dopant.

Although the data shown in FIG. 9 was obtained from the n-type dopedlayer, a similar impurity concentration profile is obtainable even froma p-type doped layer containing aluminum or the like as a dopant. Asshown in FIG. 8, the impurity concentration profile in the heavily dopedlayer has a generally δ-functional configuration relative to theunderlying undoped layer.

FIG. 10A shows the result of simulating a band structure at a conductionband edge in a sample A having a multilayer portion composed of fiveδ-doped layers each having a thickness of 10 nm and five undoped layerseach having a thickness of 50 nm which are alternately stacked. FIG. 10Bshows the result of simulating the distribution of carrierconcentration. FIG. 11A shows the result of simulating a band structureat a conduction band edge in a sample B having a multilayer portioncomposed of five δ-doped layers each having a thickness of 20 nm andfive undoped layers each having a thickness of 50 nm which arealternately stacked. FIG. 11B shows the result of simulating thedistribution of carrier concentration. As shown in FIG. 10A and FIG.11A, electrons are confined to a V-shaped Coulomb potential (quantumwell) sandwiched between donor layers each positively charged in a crosssection orthogonally intersecting the δ-doped layers and a quantum stateis formed within the well. The effective mass of an electron is 1.1 andthe relative dielectric constant of the 6H—SiC layer is 9.66. Abackground carrier concentration in the 6H—SiC layer used as the undopedlayer is 1×10¹⁵ cm⁻³ and the density of carriers in the n-type δ-dopedlayer is 1×10¹⁸ cm⁻³.

As shown in FIG. 10B, two-dimensional electrons are distributedextensively even in the undoped layer sandwiched between the two δ-dopedlayers (sample A) each having a thickness of 10 nm and the region inwhich the concentration of electrons is 2×10¹⁶ cm⁻³ or more is observedin the range at 25 nm from the interface. This indicates that thedistribution of carriers shown in FIG. 10B is coincident with the stateof carrier distribution diagrammatically depicted in FIG. 8A and thatcarriers have spread out from the δ-doped layers to the undoped layer.

On the other hand, a large overlapping portion exists between a regionin which the probability of presence of a carrier defined by the wavefunction of an electron is high and each of the δ-doped layers (sampleB) having a large thickness of 20 nm and the center of ionized impurityscattering so that the region in which the electron concentration is2×10¹⁶ cm⁻³ or more is at 11 nm from the interface, as shown in FIG.11B. This indicates that a relatively small number of carriers havespread out from the δ-doped layers to the undoped layers. In this casealso, however, the fundamental effects achieved by the multiple δ-dopedlayer according to the present invention are achievable if the minimumvalue of the carrier concentration in the region between the δ-dopedlayers is higher than the original carrier concentration in the undopedlayer. The effects achieved by the spreading out of carriers can beadjusted appropriately by adjusting the impurity concentration and filmthickness of each of the δ-doped layers and the undoped layers.

Since the present embodiment has provided, on the InP substrate 10, themultiple δ-doped InGaAs layer 12 and the multiple δ-doped InAlAs layer13 having the structure shown in the lower part of FIG. 7, the followingremarkable effects are achievable in each of the elements.

Schottky Diode

First, in the Schottky diode 20, carriers in the n-type doped layers 12a are so distributed as to spread out even to the undoped layers 12 bunder a quantum effect. If a forward bias is applied to the Schottkydiode 20 in this state, the potential of the multiple δ-doped InGaAslayer 12 is increased and electrons are constantly supplied to then-type doped layers 12 a and to the undoped layers 12 b, so that acurrent easily flows in the Schottky electrode 21 through the n-typedoped layers 12 a and undoped layers 12 b of the multiple δ-doped InGaAslayer 12. In short, not only the n-type doped layers 12 a of themultiple δ-doped InGaAs layer 12 but also the undoped layers 12 bthereof function as the carrier flow region. Since the impurityconcentration in each of the undoped layers 12 b is low, impurityscattering is reduced in the undoped layers 12 b. This retains a lowresistance, while achieving low power consumption and a large electriccurrent. If a reverse bias is applied to the Schottky diode 20, on theother hand, the depletion layer expands from the undoped layers 12 b ofthe multiple δ-doped InGaAs layer 12 to the n-type doped layers 12 athereof so that the entire multiple δ-doped InGaAs layer 12 is depletedeasily and a high breakdown voltage is obtained. Accordingly, a powerdiode with high power and a high breakdown voltage can be implemented.

A detailed description will be given herein below to the effects of thelateral Schottky diode according to the present embodiment in comparisonwith those of the conventional vertical Schottky diode.

FIG. 12A1 to FIG. 12C3 are energy band diagrams each showing changes inthe configuration of a conduction band edge caused by changes in bias inthe Schottky diode according to the present embodiment and in theconventional Schottky diode. FIG. 12A1, FIG. 12B1, and FIG. 12C1 showconduction band edges in the undoped layer 12 b of the Schottky diodeaccording to the present embodiment. FIG. 12A2, FIG. 12B2, and FIG. 12C2show conduction band edges in the n-type doped layer 12 a of theSchottky diode according to the present embodiment. FIGS. 12A3, 12B3,and 12C3 show conduction band edges in the InGaAs layer of theconventional Schottky diode. It is to be noted that the conventionalSchottky diode is assumed to have a vertical configuration in which auniformly doped layer doped with nitrogen at a uniform concentration isin contact with the Schottky electrode and an ohmic electrode is inohmic contact with any portion of the uniformly doped layer. FIG. 12A1to FIG. 12A3 show the configurations of the conduction band edges whenno voltage is applied between the Schottky electrode and the ohmicelectrode (zero bias). FIG. 12B1 to FIG. 12B3 show the configurations ofthe conduction band edges when a voltage is applied between the Schottkyelectrode and the ohmic electrode such that the Schottky electrode has ahigher voltage (forward bias). FIG. 12C1 to FIG. 12C3 show theconfigurations of the conduction band edges when a voltage is appliedbetween the Schottky electrode and the ohmic electrode such that theohmic electrode has a higher voltage (reverse bias). Since the state ofcontact between the ohmic electrode and the multiple δ-doped InGaAslayer 12 does not essentially change in response to a change in bias,the depiction thereof is omitted. Since the present embodiment hasdescribed the case where the n-type semiconductor layer in whichelectrons flow as carriers is provided, the depiction of theconfiguration of a valence band edge is omitted.

As shown in FIG. 12A1 to FIG. 12A3, respective high Schottky barriers(about 1 to 2 eV) are formed between the undoped layer or the n-typedoped layer in the active region and the Schottky electrode and betweenthe uniformly doped” layer and the Schottky electrode in the state ofzero bias in each of the Schottky diode of the present embodiment andthe conventional Schottky diode.

When a forward bias is applied to the Schottky diode of the presentembodiment, the potential of the multiple δ-doped InGaAs layer 12 isincreased, as shown in FIG. 12B1 and FIG. 12B2. In other words, theenergy level at a conduction band edge in each of the undoped layer 12 bof the multiple δ-doped InGaAs layer 12 and in the n-type doped layer 12a thereof is increased. Since a distribution of carriers as shown inFIG. 8A is formed also in the undoped layer 12 b, a current easily flowsin the Schottky electrode 21 through the n-type doped layers 12 a andthe undoped layers 12 b in the multiple δ-doped InGaAs layer 12. Inshort, not only the n-type doped layers 12 a of the multiple δ-dopedInGaAs layer 12 but also the undoped layers 12 b thereof function as thecarrier flow region. Although a distribution of carriers as shown inFIG. 8A is formed in the undoped layer 12 b, the impurity concentrationtherein is low so that impurity scattering is reduced in the undopedlayer 12 b. This maintains the resistance of the whole multiple δ-dopedInGaAs layer 12 at a high value and achieves lower power consumption anda large electric current.

On the other hand, if a forward bias is applied to the conventionalSchottky diode as shown in FIG. 12B3, a current flows from the uniformlydoped layer to the Schottky electrode.

If a reverse bias is applied to the Schottky diode according to thepresent embodiment as shown in FIG. 12C1 and FIG. 12C2, the overallenergy level at the conduction band edge in the undoped layers 12 b ofthe multiple δ-doped InGaAs layer 12 and in the n-type doped layers 12 athereof lowers. Thus, the breakdown voltage is defined by an electricfield applied to the depletion layer in the reverse bias state. In thatcase, the slope of the conduction band edge is gentler as the impurityconcentration is lower so that the width of the depletion layer isnaturally wider as the impurity concentration is lower. Accordingly, ahigh breakdown voltage is obtained in the undoped layer 12 b as shown inFIG. 12C1. If the heavily doped layer and the Schottky electrode are inmere contact, the conduction band edge in the heavily doped layer in thereverse bias state is as indicated by the broken curve in FIG. 12C2 andthe width of the depletion layer in the heavily doped layer is reducedsignificantly. However, since the thickness of the n-type doped layer 12a is as small as 10 nm in the present embodiment, the depletion layerexpands from the undoped layer 12 b into the n-type doped layer 12 a asindicated by the solid curve in FIG. 12C2.

If the whole multiple δ-doped InGaAs layer 12 is depleted, carriers arenot distributed in the undoped layer 12 b so that the entire multipleδ-doped InGaAs layer 12 is increased in resistance. If depletion isincomplete, an electric current flowing from the Schottky electrode 21toward the withdrawn doped layer 22 experiences a high resistance in then-type doped layer 12 a because of the thickness of the n-type dopedlayer 12 a which is as small as 10 nm, so that it does not flowactually. In short, substantially no ohmic contact is provided betweenthe n-type doped layer 12 a and the Schottky electrode 21 so thatSchottky contact is retained. By adjusting the thicknesses, impurityconcentrations, and the like of the undoped layers 12 b and the n-typedoped layers 12 a, the breakdown voltage can be defined by the width ofthe depletion layer between the thicker undoped layer 12 b and theSchottky electrode 21. Thus, a high breakdown voltage is obtained.

Since the width of the depletion layer in the uniformly doped layervaries with the impurity concentration of the uniformly doped layer inthe conventional Schottky diode as shown in FIG. 12C3, it is possible tocontrol the resistance and the breakdown voltage by adjusting theimpurity concentration of the uniformly doped layer. However, atrade-off exists between a low resistance and a high breakdown voltagesuch that, if the impurity concentration of the uniformly doped layer isincreased to lower the resistance, the width of the depletion layer isreduced and the breakdown voltage lowers, while the resistance increasesif the impurity concentration of the uniformly doped layer is lowered.In the conventional Schottky diode, therefore, it is difficult tosimultaneously achieve a low resistance (low power consumption) and ahigh breakdown voltage which are desired for a power device. If alateral configuration is used for the conventional Schottky diode, onthe other hand, it is difficult to achieve a high breakdown voltage,while providing a large current. So far, the conventional Schottky diodehas been implemented only in a vertical configuration.

In the Schottky device according to the present embodiment, by contrast,carriers are distributed extensively over the n-type doped layers(heavily doped layers) 12 a and the undoped layers (lightly dopedlayers) 12 b in the forward bias state and impurity scattering isreduced in the undoped layers 12 b. This allows easy movement ofcarriers (electrons) from the withdrawn doped layer 22 to the Schottkyelectrode 21. Conversely, carriers are not present in the undoped layers12 b in the reverse bias state as a result of depletion so that theentire active region is increased in resistance and electrons do notsubstantially flow from the Schottky electrode 21 to the withdrawn dopedlayer 22. By thus focusing attention on the state of carrierdistribution which is different in the forward bias state and in thereverse bias state, the trade-off between a low resistance and a highbreakdown voltage observed in the conventional Schottky diode can beeliminated with the MESFET according to the present embodiment.

The power diode thus formed in the lateral configuration facilitates theintegration of the power diode in conjunction with a power MESFET andthe like in a common InP substrate. Since it is difficult to achieve ahigh breakdown voltage, while providing a large current in theconventional Schottky diode formed in a lateral configuration, ahigh-power Schottky diode should inevitably be formed in a verticalconfiguration. By contrast, the Schottky diode according to the presentembodiment can be used also as a power device by eliminating thetrade-off between a low resistance and a high breakdown voltage andproviding a large amount of current. If an integrated circuit device isconstructed by integrating the Schottky diode according to the presentembodiment in conjunction with a HEMT, a MESFET, and the like in acommon InP substrate, the integrated circuit device can be used forequipment for a communication system. In that case, prominent effectsare achieved such that impedance matching in equipment for acommunication system handling an RF signal is performed more easily thanin the vertical Schottky diode of discrete type and the operatingfrequency is increased.

The vertical Schottky diode having a capacitor structure isdisadvantageous in that the operating frequency is lowered by aparasitic capacitance. By contrast, the lateral Schottky diode accordingto the present embodiment has no capacitor structure so that it offersthe advantage of a further increase in operating frequency.

In conventional equipment for a communication system such as a basestation, a diode is provided on a silicon substrate. In that case, not aSchottky diode but a pin diode or a pn diode is formed normally in termsof the characteristics of silicon. By using an InP substrate, however, aSchottky diode can be formed easily as in the present embodiment. Sincethe Schottky diode is characterized in that a carrier requires a shortertime to recover than in the pin diode or pn diode, a structure suitablefor higher-speed operation is obtainable.

MESFET

In the MESFET 30, carriers in the n-type doped layers 12 a are sodistributed as to spread out to the undoped layers 12 b under a quantumeffect, similarly to the Schottky diode 20. If a forward bias is appliedto the MESFET 30 in this state, the potential of the multiple δ-dopedInGaAs layer 12 is increased and electrons are supplied constantly tothe n-type doped layers 12 a and to the undoped layers 12 b.Consequently, a current easily flows between the source and drainelectrodes through both of the n-type doped layers 12 a and the undopedlayers 12 b in the multiple δ-doped InGaAs layer 12. Since the impurityconcentration in each of the undoped layers 12 b is low, impurityscattering in the undoped layers 12 b is reduced so that a lowresistance is retained and low power consumption and a large current areachieved.

When the MESFET is in the OFF state, a depletion layer expands from theundoped layers 12 b to the n-type doped layers 12 a in the multipleδ-doped InGaAs layer 12 so that the whole multiple δ-doped InGaAs layer12 is easily depleted and a high breakdown voltage is obtained.Consequently, a device for a power amplifier having a low ON-stateresistance, high power, and a high breakdown voltage is obtainable.

A description will be given herein below to the result of evaluating theperformance of the MESFET according to the present embodiment and to acomparison made between the performance of the MESFET according to thepresent embodiment and the performance of a conventional MESFET based onitems disclosed in the PCT application (PCT/JP00/01855) by the presentinventors, which are related to the SiC layer.

First, the MESFET according to the present embodiment and theconventional MESFET were compared for a gate-to-source breakdownvoltage. The MESFET having the multiple δ-doped layer using, as thechannel layer, the active region formed by alternately stacking the fiveundoped SiC layers and the five n-type doped SiC layers in the SiC layerhad a dielectric breakdown voltage of 120 V, which is four times thebreakdown voltage of the conventional MESFET.

Next, the dependence of the relationship between drain current and drainvoltage on gate voltage (I-V characteristic) was examined for theSIC-MESFET having the multiple δ-doped layer. By applying a voltage tothe gate electrode with the application of a constant voltage betweenthe source electrode and the drain electrode, a source-to-drain currentwas modulated in accordance with the voltage applied to the gateelectrode so that a switching operation was performed. At that time, astable drain current was obtained without a breakdown even when thedrain voltage was 140 V or higher.

FIG. 13 shows the result of measuring the dependence of the relationshipbetween drain current and drain voltage on gate voltage (I-Vcharacteristic) for the MESFEAT having the multiple δ-doped layer. Inthe drawing, the horizontal axis represents the drain voltage Vds (V)and the vertical axis represents the drain current Ids (A), while thegate voltage Vg is used as a parameter.

Further, a transconductance in the vicinity of the threshold voltage wasmeasured for each of the MESFET having the multiple δ-doped layer andthe conventional MESFET. As a result, it was found that thetransconductance of the MESFET using the foregoing multiple δ-dopedlayer as the channel layer was about double the transconductance of theconventional MESFET using the uniformly doped layer as the channellayer. This is because the mobility of an electron in the MESFET havingthe multiple δ-doped layer was increased as described above.

In accordance with the same principle, the foregoing results are alsoobtainable from the MESFET according to the present embodiment havingthe multiple δ-doped InGaAs layer 12 provided on the InP substrate.Therefore, the MESFET according to the present embodiment can achievethe effects of low power consumption, a high breakdown voltage, and ahigh gain.

If a comparison is made between the power amplifier according to thepresent embodiment and a conventional power amplifier the followingdifferences are observed therebetween in correspondence with theforegoing functional differences between the MESFET according to thepresent embodiment and the conventional MESFET.

At the conventional base station, the amplified-signal transmitter whichrequires amplification of high power has four main amplifiers eachcomprising a MESFET placed therein, as shown in FIG. 27. However, itbecomes more difficult to achieve impedance matching between theindividual MESFETs as a larger number of MESFETs are provided and thedifficulty of impedance matching increases as the frequency of an RFsignal is higher.

By contrast, the present embodiment can obtain a desired power by merelydisposing one main amplifier 138 comprising a MESFET in theamplified-signal transmitting circuit. By thus reducing the number ofMESFETs, the structure of an impedance matching circuit can besimplified in a circuit handling an RF signal in a high frequency rangecompared with a circuit in the conventional base station. In addition,the Schottky diodes are integrated in conjunction with the MESFETs inthe single InP substrate, as described above, and the number thereof isreduced. This further simplifies the structure of the impedance matchingcircuit and allows a semiconductor integrated circuit device in whichMESFETs according to the present embodiment are mounted to beincorporated into a communication system handling an RF signal on, e.g.,the GHz order.

FIG. 29 shows the characteristics of a power amplifier determined asspeculations for a mobile data terminal. The characteristics shown inFIG. 29 are an input-power/output-power characteristic, an efficiencycharacteristic, and a distortion characteristics for a GaAs-MESFET. Asshown in the drawing, an output power can be divided into a linearregion R1 where the efficiency is low but linearity is retained, aquasi-linear region R2 where the efficiency is relatively high, and asaturation region R3 where the output power is substantially saturated.In a PDC system using a narrow band of frequencies also, the poweramplifier is used in the quasi-linear region R2 where the efficiency isrelative high. In a W-CDMA system using a wide band of frequencies, thepower amplifier is used in the linear region R1 where the efficiency islow. In general, the average power of a radio wave to be transmitted anda peak power has a large difference therebetween. To reduce distortion,therefore, the linear region R1 should be large.

As shown in FIG. 13, the SiC-MESFET having the multiple δ-doped layerhas the drain current Ids which is high in saturation value and thedrain voltage Vd which is high in breakdown value. Since the power P canbe calculated by dividing the product of the drain current Ids and thedrain voltage Vd by 8, the SiC-MESFET shown in FIG. 13 has the largelinear region RI due to the high saturation value of the drain currentand the high breakdown voltage. This allows the retention of linearityup to an output power as high as about 1 W. Accordingly, the linearregion can be used in the PDC system, while the high efficiency isretained. In addition, the region where the efficiency lowers and theregion where the distortion is increased shift to higher output powers.Since the function obtained by providing the multiple δ-doped layer inthe MESFET according to the present embodiment is nearly the same as thefunction obtained in the SiC-MESFET, the same effects achieved by theSiC-MESFET can be expected.

HEMT

In the HEMT 40, the band gap of In_(0.52)Al_(0.48)As as the secondactive region is larger than that of In_(0.53)Ga_(0.47)As as the firstactive region, as shown in FIG. 18. Consequently, a discontinuity occursbetween the respective conduction band edges in the multiple δ-dopedInAlAs layer 13 and the InGaAs layer 17 so that a so-calledheterobarrier is formed. When a voltage Vg is applied to the Schottkygate electrode 42, a depressed portion to which a two-dimensionalelectron gas is confined is formed in the region of the InGaAs layer 17adjacent the interface between the InGaAs layer 17 and the multipleδ-doped InAlAs layer 13 due to a bend in the band as indicated by thebroken line in the drawing. As a result, electrons are allowed to moveat a high speed in the depressed portion. Since the present embodimenthas provided the InAlAs layer 16 (in which ratios of components areIn_(0.52)Al_(0.48)As) under the InGaAs layer 17, a discontinuity(heterobarrier) also occurs between the respective conduction band edgesin the InGaAs layer 17 and the InAlAs layer 16. This allows morepositive confinement of electrons to the InGaAs layer 17. The underlyingInAlAs layer 16 need not necessarily be provided.

In the HEMT 40, a current from the drain electrode 45 passes through theInP layer 18 and the multiple δ-doped InAlAs layer 13 to flow in theregion (channel region) of the InGaAs layer 17 adjacent the interfacebetween the InGaAs layer 17 and the multiple δ-doped InAlAs layer 13.Thereafter, the current flows to the source electrode 44 after passingthrough the multiple δ-doped InAlAs layer 13 and the InP layer 18.

In the multiple δ-doped InAlAs layer 13 according to the presentembodiment, carriers in the n-type doped layers 13 a are so distributedas to spread out extensively to the undoped layers 13 b under a quantumeffect. If a bias is applied to the HEMT 40 in this state, carriers(electrons) are supplied to the channel layer of the InGaAs layer 17through both of the n-type doped layers 13 a and undoped layers 13 b ofthe multiple δ-doped InAlAs layer 13, so that a large current flows inthe channel region. Since the impurity concentration in each of theundoped layers 13 b is low, impurity scattering is reduced in theundoped layer 13 b. Accordingly, the resistance when carriers aresupplied from the multiple δ-doped InAlAs layer 13 to the channel regionis held low and the efficiency with which carriers are supplied isenhanced. This implements a HEMT with low power consumption and a largecurrent.

When the HEMT 40 is in the OFF state, depletion layers expand from theundoped layers 13 b to the n-type doped layers 13 a in the multipleδ-doped InAlAs layer 13 so that the whole multiple δ-doped InAlAs layer13 is depleted easily. This achieves a high breakdown voltage.

In the I-V characteristic (Drain Current Id−Drain voltage VdCharacteristic), therefore, the respective threshold values of the draincurrent Id and the drain voltage Vd (current and voltage values at abreakdown point) can be increased. Since the power P of the HEMT isrepresented by the following expression:

 P=Iv×Vd/8,

a HEMT with a particularly high power is obtainable. In short, a devicefor a power amplifier having a low ON-state resistance, a high power,and a high breakdown voltage can be obtained.

If a comparison is made between the power amplifier according to thepresent embodiment in which the HEMT is disposed and the conventionalpower amplifier they have the following differences therebetween. At theconventional base station, the amplified-signal transmitter whichrequires amplification of high power has four main amplifiers eachcomprising a HEMT or a MESFET disposed therein, as shown in FIG. 27. Asa larger number of HEMTs or MESFETs are provided, however, it becomesmore difficult to achieve impedance matching between the individualHEMTs or between the individual MESFETs and the difficulty of impedancematching increases as the frequency of an RF signal is higher.

By contrast, the present embodiment can provide a desired power bymerely disposing one main amplifier 138 comprising a HEMT in theamplified-signal transmitting circuit. By thus reducing the number ofHEMTs, the structure of an impedance matching circuit can be simplifiedin a circuit handling an RF signal in a high frequency range comparedwith a circuit in the conventional base station.

If each of the HEMT 40 and the MESFET 30 is provided, either of thedevices can be used selectively depending on a frequency region.Specifically, the HEMT can be used selectively to amplify an RF signalin a frequency region on the millimeter-wave level. To amplify an RFsignal in the lower frequency region, the MESFET can be usedselectively.

If the BST film is formed to occupy an area which is, e.g., 5 mm square,the capacitor 50 provides a capacitance of about 22 μF since the BSTfilm has a relative dielectric constant of about 1000 and the thicknessthereof can be reduced to about 10 nm. In short, a capacitor occupying asmall area and having a large capacitance can be formed.

If a conductor film having a spiral configuration with a line width of 9μm is provided with a 4-μm spacing in an area which is about 5 mmsquare, the inductor 60 has about 160 turns and an inductance of 780 μH.In short, an inductor occupying a small area and satisfying desiredspecifications can be provided.

Each of the multilayer portions in the Schottky diode, the MESFET, andthe HEMT may have only one heavily doped layer and only one lightlydoped layer. Either one of the heavily doped layer and the lightly dopedlayer may be formed previously to the formation of the other. It is alsopossible to place two lightly doped layers (undoped layers) above andbelow one heavily doped layer. That is, the number of heavily dopedlayers may be different from the number of lightly doped layers.

Referring to FIG. 14A to FIG. 16B, the process for fabricating thesemiconductor device according to the present embodiment will bedescribed. FIG. 14A to FIG. 14C are cross-sectional views illustratingthe process steps of fabricating the semiconductor device according tothe present embodiment from the formation of the multiple δ-doped InGaAslayer, the multiple δ-doped InAlAs layer, and the like to the formationof an isolation region. FIG. 15A and FIG. 15B are cross-sectional viewsillustrating the process steps of fabricating the semiconductor deviceaccording to the present embodiment from the formation of the respectiveelectrodes of individual elements to the formation of a conductor film.FIG. 16A and FIG. 16B are cross-sectional views illustrating the processsteps of fabricating the semiconductor device according to the presentembodiment from the formation of the upper electrodes of capacitors tothe formation of contact holes connecting to the respective conductorportions of the elements.

First, in the step shown in FIG. 14A, the semi-insulating InP substrate10 doped with iron (Fe) at a high concentration and having a thicknessof about 100 μm is prepared. After a surface of the InP substrate 10 iscleaned, the InP substrate 10 is placed in the chamber of an MBE system(molecular-beam epitaxial growth system). Then, source material beams ofIn, Al, and As for forming the InAlAs layer are supplied into thechamber to form the undoped InAlAs layer 15 (in which ratios ofcomponents are, e.g., In_(0.52)Al_(0.48)As).

Next, the supply of Si is controlled through the opening and closing ofthe shutter of a dopant supply unit, while source material beams forforming the InGaAs layer are supplied, whereby the multiple δ-dopedInGaAs layer 12 (In_(0.53)Ga_(0.47)As layer) with a thickness of about70 nm is formed by the following procedure.

First, the supply of the dopant is halted, while the source materialbeams are supplied to the chamber. Specifically, the source materialbeams are supplied to a space above the InP substrate 10 with theshutter closed completely, whereby the undoped layer (lightly dopedlayer) 12 b composed of an undoped InGaAs single crystal and having athickness of about 10 nm is grown epitaxially on a principal surface ofthe InP substrate 10 in the chamber.

Next, Si (dopant) as an n-type impurity is supplied to the chamber,while the source material beams are supplied thereto, whereby the n-typedoped layer (heavily doped layer) 12 a composed of an InGaAs singlecrystal and having a thickness of about 1 nm is grown epitaxially on theundoped layer 12 b in the chamber.

When the epitaxial growth of the n-type doped layer 12 a is completed,the source material beams are supplied to the space above the InPsubstrate 10 by halting the supply of the dopant, i.e., with the shutterclosed completed, whereby the undoped layer (lightly doped layer) 12 bcomposed of an undoped InGaAs single crystal and having a thickness ofabout 10 nm is grown epitaxially on the principal surface of the InPsubstrate 10.

Each of the step of forming the n-type doped layer 12 a by thusintroducing the dopant through the opening and closing of the shutter,while simultaneously supplying the source material beams, and the stepof forming the undoped layer 12 b by thus supplying only the sourcematerial beams without supplying the dopant, while keeping the shutterclosed, is repeated five times. As the uppermost layer, the undopedlayer 12 b is formed again to have a thickness larger by about 5 nm thanthat of each of the other undoped layers 12 b. By the foregoing process,the multiple δ-doped InGaAs layer 12 composed of the n-type doped layers12 a and the undoped layers 12 b which are stacked alternately isformed.

After the undoped layer 12 b composed of InGaAs with a thickness ofabout 10 nm is formed as the uppermost layer of the multiple δ-dopedInGaAs layer 12, an InP layer with a thickness of about 5 nm may also begrown epitaxially as an etching stopping layer.

Next, the undoped InAlAs layer 16 (in which ratios of components areIn_(0.52)Al_(0.48)As) with a thickness of about 10 nm is formed on themultiple δ-doped InGaAs layer 12 by switching the source material beams.Then, the undoped InGaAs layer 17 (in which ratios of components areIn_(0.53)Ga_(0.47)As) with a thickness of about 10 nm is formed on theInAlAs layer 17 by switching the source material beams.

Thereafter, the five n-type doped layers (heavily doped layers) 13 aeach having a thickness of about 1 nm and the six undoped layers 13 beach having a thickness of about 10 nm are stacked by supplying thesource material beams for forming the InAlAs layer and controlling theopening and closing of the shutter, similarly to the foregoing procedureof forming the multiple δ-doped InGaAs layer 12, whereby the multipleδ-doped InAlAs layer 13 with a thickness of about 65 nm is formed. Atthis time, the undoped layers 13 b are formed as the uppermost andlowermost layers.

Then, the InP layer 18 composed of an InP single crystal and having athickness of about 5 nm is grown epitaxially as the etching stoppinglayer on the multiple δ-doped InAlAs layer 13 by switching the sourcematerial beams.

Next, in the step shown in FIG. 14B, the regions of the InP layer 18,the multiple δ-doped InAlAs layer 13, the InGaAs layer 17, and theInAlAs layer 16 in which the Schottky diode 20 and the MESFET 30 are tobe formed are removed by selective etching so that the multiple δ-dopedInGaAs layer 12 is exposed in each of the regions in which the Schottkydiode 20 and the MESFET 30 are to be formed.

Next, in the step shown in FIG. 14C, trenches for forming the isolationregions are formed in the substrate and a silicon oxide film is buriedin each of the trenches to form the isolation regions 11.

Next, in the step shown in FIG. 15A, the electrode withdrawn layer 22for the Schottky diode 20 is formed by implanting an n-type impurity(e.g., silicon ions Si⁺). At this time, an implant mask composed of asilicon dioxide film or the like which covers the region other than theregion in which the n-type impurity ions are to be implanted and has anopening corresponding to the region in which the n-type impurity ionsare to be implanted is formed on the substrate. Then, silicon ions (Si+)or the like are implanted from above the implant mask. Further,annealing for activating the impurity is performed at 800° C. for 10minutes, whereby the electrode withdrawn layer 22 containing the n-typeimpurity at a concentration of about 1×10¹⁸ atoms cm⁻³ is formed. Atthis time, silicon ions (Si⁺) are implanted into the substrate in, e.g.,six steps of ion implantation using different implant energies. Forexample, the first ion implantation is performed with an accelerationvoltage of 180 keV and at a dose of 1.5×10¹⁴ atoms cm⁻², the second ionimplantation is performed with an acceleration voltage of 130 keV and ata dose of 1×10¹⁴ atoms cm⁻², the third ion implantation is performedwith an acceleration voltage of 110 keV and at a dose of 5×10¹³ atomscm⁻², the fourth ion implantation is performed with an accelerationvoltage of 100 keV and at a dose of 8×10¹³ atoms cm⁻², the fifth ionimplantation is performed with an acceleration voltage of 60 kev and ata dose of 6×10¹³ atoms cm⁻², and the sixth ion implantation is performedwith an acceleration voltage of 30 kev and at a dose of 5×10¹³ atomscm⁻². In any of the six ion implantations, the direction in which ionsare implanted is 7. tilted from a normal to the InP substrate 10 and theimplant depth is about 0.3 μm.

After the implanted mask is removed, a SiN film with a thickness ofabout 0.4 μm is formed by plasma CVD on the substrate and patterned toform the underlying insulating film 51 and the dielectric film 61 on therespective regions of the multiple δ-doped InAlAs layer 13 on which thecapacitor 50 and the inductor 60 are to be formed.

Next, in the step shown in FIG. 15B, a TiPtAu film is vapor-deposited oneach of the respective regions of the multiple δ-doped InGaAs layer 12on which the Schottky diode and the MESFET are to be formed and theregion of the InP layer 18 on which the HEMT is to be formed. Then,annealing is performed at 300° C. for 3 minutes, whereby the Schottkyelectrode 21 composed of the TiPtAu film and the Schottky gateelectrodes 32 and 42 each composed of the TiPtAu film and having a gatelength of about 1 μm are formed. Then, a TiPtAu film is formed by vacuumvapor deposition on each of the MESFET formation region and the HEMTformation region, whereby the source electrodes 34 and 44 and the drainelectrodes 35 and 45 are formed. At this time, the TiPtAu film is alsovapor-deposited simultaneously on the withdrawn electrode layer 22 ofthe Schottky diode 20 so that the ohmic electrode 23 composed of theTiPtAu film is formed. On the other hand, platinum (Pt) isvapor-deposited on the underlying insulating film 51 of the capacitor 50so that the lower electrode 52 composed of platinum is formed.

Next, a resist film having a spiral opening is formed on the region onwhich the inductor 60 is to be formed. A Cu film with a thickness ofabout 4 μm is deposited on the resist film and lifted off, whereby thespiral conductor film 62 is left on the dielectric film 61. It is alsopossible to compose the conductor film of an aluminum alloy film insteadof the Cu film. In that case, the aluminum alloy film is deposited andpatterned by RIE dry etching using a Cl₂ gas and a BCl₃ gas, therebyforming the spiral conductor film 62.

Next, in the step shown in FIG. 16A, a BST film is formed by sputteringon the lower electrode of the capacitor 50. Then, a platinum (Pt) filmis formed by vapor deposition on the BST film. The platinum film and theBST film are patterned into a specified configuration to form the upperelectrode 54 and the capacitance insulating film 53.

Next, in the step shown in FIG. 16B, the interlayer insulating film 70composed of a silicon dioxide film is deposited on the substrate. Theinterlayer insulating film 70 is formed with contact holes 74 reachingthe Schottky electrode 21 and ohmic electrode 23 of the Schottky diode20, the Schottky gate electrode 32 and source and drain electrodes 34and 35 of the MESFET 30, the gate, source, and drain electrodes 42, 44,and 45 of the HEMT 40, the upper and lower electrodes 54 and 52 of thecapacitor 50, and the center and outer circumferential end portions ofthe spiral conductor film 62 of the inductor 60.

Thereafter, an aluminum alloy film is formed in each of the contactholes 72 and on the interlayer insulating film 70 and patterned toprovide the structure of the semiconductor device shown in FIG. 7.

Although the InGaAs layer and the InAlAs layer have been grownepitaxially by using an MBE technique in the foregoing description ofthe fabrication process, the InGaAs layer and the InAlAs layer may alsobe grown epitaxially by using a MOCVD technique.

Thus, the fabrication method according to the present embodiment allowseasy formation of the Schottky diode, the MESFET, the HEMT, the resistorelement, the inductor, and the like on the single InP substrate. Sincethe HEMT, the MESFET, and the Schottky diode are allowed to be providedon the common InP substrate by forming the Schottky diode in the lateralconfiguration, as described above, integration is particularlyfacilitated. Since the passive element such as the inductor is alsoallowed to be mounted on the common SiC substrate, further scaling downis achievable.

Although the present embodiment has used the InP substrate, the presentembodiment is applicable not only to a semiconductor device provided onthe InP substrate but also to all semiconductor devices provided on asubstrate composed of GaAs, GaN, AlGaAs, AlGaN, SiGe, SiGeC, or thelike. In that case also, the provision of the multilayer portioncomposed of the δ-doped layers and the lightly doped layers (includingundoped layers) allows improvements in channel mobility and breakdownvoltage by using reduced scattering by impurity ions and depletion ofthe whole channel region in the OFF state.

Example 1 of Specific Structure of HEMT

FIG. 17 is a schematic cross-sectional view showing a specific structureof the HEMT in a first example of the embodiment of the presentinvention. As shown in the drawing, an undoped InAlAs layer 202 (inwhich ratios of components are In_(0.52)Al_(0.48)As) with a thickness ofabout 200 nm, an undoped InGaAs layer 203 (in which ratios of componentsare In_(0.53)Ga_(0.47)As) with a thickness of about 15 nm, a multipleδ-doped InAlAs layer 204 (in which ratios of components areIn_(0.52)Al_(0.48)As) composed of five n-type doped layers 204 a(containing Si as an impurity) each having a thickness of about 1 nm andsix undoped layers 204 b each having a thickness of about 10 nm whichare alternately stacked (of which the uppermost and lowermost layers areundoped) to serve as a carrier supplying layer with a thickness of about65 nm, an InP layer 205 serving as an etching stopping layer with athickness of about 5 nm, an n-InAlAs layer 206 (in which ratios ofcomponents are In_(0.52)Al_(0.48)As) doped with silicon (Si) and havinga thickness of about 3 nm, an n⁺-InAlAs layer 207 (in which ratios ofcomponents are In_(0.52)Al_(0.48)As) doped with silicon (Si) as ann-type impurity at a high concentration and having a thickness of about200 nm, and an n⁺-InGaAs layer 508 (in which ratios of components areIn_(0.53)Ga_(0.47)As) doped with silicon (Si) as an n-type impurity at ahigh concentration and having a thickness of about 15 nm are stackedsuccessively on a semi-insulating InP substrate 201 doped with iron (Fe)at a high concentration and having a thickness of about 100 μm. Thereare also provided ohmic source/drain electrodes 209 a and 209 b whichare composed of a Ti/Pt/Au film and provided in mutually spaced relationon the n⁺-InGaAs layer 208, a Schottky gate electrode 210 composed of aWSiN film 210 a which penetrates respective parts of the n-InAlAs layer206, the n⁺-InAlAs layer 207, and the n⁺-InGaAs layer 208 to be incontact with the InP layer 205 and of an overlying Ti/Pt/Au film 210 b,and an insulating layer 211 composed of a SiO₂/SiNi film for providing adielectric isolation between the Schottky electrode 210 and thesource/drain electrodes 209 a and 209 b.

In the HEMT, if a voltage is applied between the source and drainelectrodes 209 a and 209 b, a current flows between the source and thedrain. If a voltage is applied between the Schottky gate electrode 210and the source electrode 209 a such that the Schottky gate electrode 210has a higher voltage (reverse voltage), the source-to-drain current ismodulated in accordance with the voltage applied to the Schottky gateelectrode 210 so that a switching operation is performed.

FIG. 18A and FIG. 18B are energy band diagrams schematically showing therespective states of a band during no application of a bias to aheterojunction portion in the HEMT of the present example and during theapplication thereof.

As shown in FIG. 18A, since the band gap of In_(0.52)Al_(0.48)As islarger than that of In_(0.53)Ga_(0.47)As during no application of abias, a discontinuity occurs between the respective conduction bandedges of the multiple δ-doped InAlAs layer 204 and the InGaAs layer 203so that a so-called heterobarrier is formed. When a voltage Vg isapplied to the Schottky gate electrode 210 (during the application of abias), a depressed portion to which a two-dimensional electron gas isconfined is formed in the region of the InGaAs layer 203 adjacent theinterface between the InGaAs layer 203 and the multiple δ-doped InAlAslayer 204 due to a bend in the band as shown in FIG. 18B. As a result,electrons are allowed to move at a high speed in the depressed portion.Since the present embodiment has provided the InAlAs layer 202 (in whichratios of components are In_(0.52)Al_(0.48)As) under the InGaAs layer203, a discontinuity (heterobarrier) also occurs between the respectiveconduction band edges of the InAlAs layer 202 and the InGaAs layer 203.This allows more positive confinement of electrons to the InGaAs layer203. The underlying InAlAs layer 202 need not necessarily be provided.

In the HEMT, a current from the drain electrode 209 b passes through theindividual layers 208, 207, 206, and 205 and the multiple δ-doped InAlAslayer 204 in this order to flow in the region (channel region) of theInGaAs layer 203 adjacent the interface between the InGaAs layer 203 andthe multiple δ-doped InAlAs layer 204. Thereafter, the current flows tothe source electrode 209 a after passing through the multiple δ-dopedInAlAs layer 204 and the individual layers 205, 206, 207, and 208 inthis order (see the broken arrow in FIG. 17).

In the multiple δ-doped InAlAs layer 204 according to the presentembodiment, carriers in the n-type doped layers 204 a are so distributedas to spread out extensively to the undoped layers 204 b under a quantumeffect. If a bias is applied to the HEMT in this state, carriers(electrons) are supplied to the channel layer of the InGaAs layer 203through both of the n-type doped layers 204 a and undoped layers 204 bof the multiple δ-doped InAlAs layer 204, so that a large current flowsin the channel region. Since the impurity concentration in each of theundoped layers 204 b is low, impurity scattering is reduced in theundoped layer 204 b. Accordingly, the resistance when carriers aresupplied from the multiple δ-doped layer 204 to the channel region isheld low and the efficiency with which carriers are supplied isenhanced. This implements a HEMT with low power consumption and a largecurrent.

When the HEMT is in the OFF state, depletion layers expand from theundoped layers 204 b to the n-type doped layers 204 a in the multipleδ-doped InAlAs layer 204 so that the whole multiple δ-doped InAlAs layer204 is depleted easily. This achieves a high breakdown voltage.

In the I-V characteristic (Drain Current Id−Drain Voltage VdCharacteristic), therefore, the saturation value of the drain current Idand the threshold value of the drain voltage Vd (voltage value at abreakdown point) can be increased. Since the power P of the HEMT isrepresented by the following expression:P=Iv×Vd/8,a HEMT with a particularly high power is obtainable. In short, a devicefor a power amplifier with a low ON-state resistance, a high power, anda high breakdown voltage can be obtained.

Since a gate capacitance can be reduced particularly in this example ofthe structure due to a T-shaped structure in which the gate length canbe minimized, the structure is suitable for a HEMT handling an RF signalin a high frequency region such as the millimeter-wave region. It isalso possible to adopt the structure of the HEMT shown in FIG. 17 to theHEMT 40 shown in FIG. 7.

In the structure of each of the HEMTs shown in FIGS. 7 and 17, theimpurity concentration in each of the five n-type doped layers 13 a (204a) of the multiple δ-doped InAlAs layer can be adjusted properlydepending on a frequency region in which it is used, an amplificationfactor, and the like such that, e.g., a profile in which theconcentration gradually decreases exponentially in the upward directionis provided.

Example 2 of Specific Structure of HEMT

FIG. 19 is a schematic cross-sectional view showing a specific structureof the HEMT in a second example of the embodiment of the presentinvention. In the present example, as shown in the drawing, an undopedInAlAs layer 202 (in which ratios of components areIn_(0.52)Al_(0.48)As) with a thickness of about 200 nm, a multipleδ-doped InAlAs layer 204″ (in which ratios of components areIn_(0.52)Al_(0.48)As) composed of five n-type doped layers 204 a(containing Si as an impurity) each having a thickness of about 1 nm andsix undoped layers 204 b each having a thickness of about 10 nm whichare alternately stacked (of which the uppermost and lowermost layers areundoped) to serve as a carrier supplying layer with a thickness of about65 nm, an undoped InGaAs layer 203′ (in which ratios of components areIn_(0.53)Ga_(0.47)As) with a thickness of about 15 nm, a multipleδ-doped InAlAs layer 204′ (in which ratios of components areIn_(0.52)Al_(0.48)As) composed of five n-type doped layers 204 a(containing Si as an impurity) each having a thickness of about 1 nm andsix undoped layers 204 b each having a thickness of about 10 nm whichare alternately stacked (of which the uppermost and lowermost layers areundoped) to serve as a carrier supplying layer with a thickness of about65 nm, an undoped InGaAs layer 203 (in which ratios of components areIn_(0.53)Ga_(0.47)As) with a thickness of about 15 nm, a multipleδ-doped InAlAs layer 204 (in which ratios of components areIn_(0.52)Al_(0.48)As) composed of five n-type doped layers 204 a(containing Si as an impurity) each having a thickness of about 1 nm andsix undoped layers 204 b each having a thickness of about 10 nm whichare alternately stacked (of which the uppermost and lowermost layers areundoped) to serve as a carrier supplying layer with a thickness of about65 nm, an InP layer 205 serving as an etching stopping layer with athickness of about 5 nm, an n-InAlAs layer 206 (in which ratios ofcomponents are In_(0.52)Al_(0.48)As) doped with silicon (Si) and havinga thickness of about 3 nm, an n⁺-InAlAs layer 207 (in which ratios ofcomponents are In_(0.52)Al_(0.48)As) doped with silicon (Si) as ann-type impurity at a high concentration and having a thickness of about200 nm, and an n⁺-InGaAs layer 208 (in which ratios of components areIn_(0.53)Ga_(0.47)As) doped with silicon (Si) as an n-type impurity at ahigh concentration and having a thickness of about 15 nm are stackedsuccessively on a semi-insulating InP substrate 201 doped with iron (Fe)at a high concentration and having a thickness of about 100 μm. Inshort, the structure of the second example is obtained by providing thestructure of the first example with two more multiple δ-doped layers.

The structures of the other components of the HEMT in the presentexample are the same as in the first example. In the HEMT of the presentexample, if a voltage is applied between the source and drain electrodes209 a and 209 b, a current flows between the source and the drain. If avoltage is applied between the Schottky gate electrode 210 and thesource electrode 209 a such that the Schottky gate electrode 210 has ahigher voltage (reverse voltage), the source-to-drain current ismodulated in accordance with the voltage applied to the Schottky gateelectrode 210 so that a switching operation is performed.

FIG. 20A and FIG. 20B are energy band diagrams schematically showing therespective states of a band during no application of a bias to aheterojunction portion in the HEMT of the present example and during theapplication thereof.

As shown in FIG. 20A, since the band gap of In_(0.52)Al_(0.48)As islarger than that of In_(0.53)Ga_(0.47)As during no application of abias, a discontinuity occurs between the respective conduction bandedges of the multiple δ-doped InAlAs layer 204 and the InGaAs layer 203,of the InGaAs layer 203 and the multiple δ-doped InAlAs layer 204′, ofthe multiple δ-doped InAlAs layer 204′ and the InGaAs layer 203′, and ofthe InGaAs layer 203′ and the multiple δ-doped InAlAs layer 204″ so thata so-called heterobarrier is formed.

When a voltage Vg is applied to the Schottky gate electrode 210 (duringthe application of a bias), a depressed portion to which atwo-dimensional electron gas is confined is formed in each of the regionof the InGaAs layer 203 adjacent the interface between the InGaAs layer203 and the multiple δ-doped InAlAs layer 204, the region of the InGaAslayer 203 adjacent the interface between the InGaAs layer 203 and themultiple δ-doped InAlAs layer 204′, the region of the InGaAs layer 203′adjacent the interface between the InGaAs layer 203′ and the multipleδ-doped InAlAs layer 204′, and the region of the InGaAs layer 203′adjacent the interface between the InGaAs layer 203′ and the multipleδ-doped InAlAs layer 204″ due to a bend in the band, as shown in FIG.20B. In short, a total of four depressed portions are formed in therespective regions of the InGaAs layers 203 adjacent the upper and lowersurfaces thereof and in the respective regions of the InGaAs layers 203′adjacent the upper and lower surfaces thereof. Each of the depressedportions functions as a channel region in which electrons move at a highspeed. Thus, in the HEMT of the present example, the first to fourthchannel regions Rch1 to Rch4 are formed as shown in FIG. 19 and hencethe HEMT of the present embodiment may be referred to as a multi-channelHEMT.

Since the total of four channel regions are formed in the multi-channelHEMT of the present example, the concentration of the two-dimensionalelectron gas is further increased (e.g., nearly fourfold) compared withthe first example having only one channel region.

In the multi-channel HEMT, a current from the drain electrode 209 bpasses through the InP layer 205 and the multiple δ-doped InAlAs layer204 and then part of the current flows in the two regions (the firstchannel region Rch1 and the second channel region Rch2) of the InGaAslayer 203 adjacent the respective interfaces between the InGaAs layer203 and the multiple δ-doped InAlAs layer 204 and between the InGaAslayer 203 and the multiple δ-doped InAlAs layer 204′. Thereafter, thecurrent flows to the source electrode 209 a after passing through themultiple δ-doped InAlAs layer 204 and the InP layer 205. The remainingcurrent passes through the multiple δ-doped InAlAs layer 204′ to flow inthe two regions (the third channel region Rch3 and the fourth channelregion Rch4) of the InGaAs layer 203′ adjacent the respective interfacesbetween the InGaAs layer 203′ and the multiple δ-doped InAlAs layer 204′and between the InGaAs layer 203′ and the multiple δ-doped InAlAs layer204″. Thereafter, the current flows to the source electrode 209 athrough the multiple δ-doped InAlAs layer 204′, the InGaAs layer 203,the multiple δ-doped InAlAs layer 204, and the InP layer 205.

In the multiple δ-doped InAlAs layers 204, 204′, and 204″ according tothe present embodiment, carriers in the n-type doped layers 204 a, 204a′ and 204″ are so distributed as to spread out extensively to theundoped layers 204 b, 204 b′, and 204″. If a bias is applied to the HEMTin this state, carriers (electrons) are supplied from the multipleδ-doped InAlAs layer 204 to the first channel region Rch1, from themultiple δ-doped InAlAs layer 204′ to the second and third channelregions Rch2 and Rch3, and from the multiple δ-doped InAlAs layer 204″to the fourth channel region Rch4. In other words, carriers (electrons)are supplied to each of the channel regions through both of the n-typedoped layers and undoped layers of any of the multiple δ-doped InAlAslayers, so that a large current flows in each of the channel regions.Since the impurity concentration in each of the undoped layers is low asdescribed in the first example, impurity scattering is reduced in theundoped layer. Accordingly, the resistance when carriers are suppliedfrom any of the multiple δ-doped layers to each of the channel regionsis held low and the efficiency with which carriers are supplied isenhanced. This implements a HEMT with low power consumption and a largecurrent.

In the multi-channel HEMT of the present example, in particular, thetotal of four channel regions are formed so that a HEMT with lower powerconsumption and a larger current (e.g., nearly fourfold) is implementedthan in the first example having only one channel region.

When the HEMT is in the OFF state, depletion layers expand from theundoped layers 204 b, 204 b′, and 204 b″ to the n-type doped layers 204a, 204 a′, and 204″ in the multiple δ-doped InAlAs layers 204, 204′, and204″ so that the whole multiple δ-doped InAlAs layers 204, 204′, and204″ are depleted easily. This achieves a high breakdown voltage.

In the I-V characteristic (Drain Current Id−Drain Voltage VdCharacteristic), therefore, the saturation value of the drain current Idand the threshold value of the drain voltage Vd (voltage value at abreakdown point) can further be increased than in the first example. Asdescribed above, since the power P of the HEMT is represented by thefollowing expression:P=Iv×Vd/8,a multi-channel HEMT with a particularly high power is obtainable. Inshort, a device for a power amplifier having a lower ON-state resistanceand a higher power than the HEMT of the first example and a breakdownvoltage equal to that of the HEMT of the first example can be obtained.

For example, a structure of a so-called double-channel HEMT in which twohigh-breakdown-voltage doped layers are provided above and below anintrinsic semiconductor layer and two channel regions are formed in theupper and lower portions of the intrinsic semiconductor layer isdisclosed in a document (HOS based algorithm for autofocusing ofspotlight SAR images F. Berizzi, G. Corsini and Gini “ELECTRONICSLETTERS 27, Mar. 1997 Vol. 33 No.7”). The double-channel HEMT is soconfigured as to supply carriers from each of the high-breakdown-voltagedoped layers to each of the channel regions. If the impurityconcentration is increased in each of the high-breakdown-voltage dopedlayers of the conventional double-channel HEMT, however, an amount ofcurrent in the channel region can be increased, while the breakdownvoltage is reduced. If the impurity concentration is reduced for ahigher breakdown voltage, on the other hand, an amount of current in thechannel region is reduced. Thus, it has been difficult to provide asufficiently high power P represented by the expression:P=Iv×Vd/8.By contrast, the multi-channel HEMT of the present example achieves anincreased amount of current and a higher breakdown voltage at the sametime so that the power is increased.

Since a gate capacitance can be reduced particularly in this example ofthe structure due to a T-shaped structure in which the gate length canbe minimized, the structure is suitable for a HEMT handling an RF signalin a high frequency region such as a millimeter-wave region. It is alsopossible to adopt the structure of the HEMT shown in FIG. 19 to the HEMT40 shown in FIG. 7.

In the HEMT shown in FIG. 19, the amounts of current in the respectivechannel regions Rch1, Rch2, Rch3, and Rch4 need not necessarily beuniform. The ratio among the amounts of current in the respectivechannel regions Rch1, Rch2, Rch3, and Rch4 can be changed variously byadjusting the respective impurity concentrations and film thicknesses ofthe undoped layers 204 b, 204 b′, and 204 b″ and the n-type doped layers204 a, 204 a′, and 204 a″ in the multiple δ-doped InGaAs layers 204,204′, and 204″, the numbers of stacked layers in the respective multipleδ-doped InGaAs layers 204, 204′, and 204″, the respective impurityconcentrations and film thicknesses in the InGaAs layers 203 and 203′,the impurity concentration and film thickness of the InP layer 205, andthe like. The ratios among the amounts of current in the respectivechannel regions can be determined selectively depending on the type of asemiconductor device in which the HEMT is used.

It has been difficult for the conventional double-channel HEMT toprovide a large linear region in an input-power/output-powercharacteristic as shown in FIG. 29, since the ratios among the amountsof current in the respective channel regions vary as the input power isincreased. By contrast, the multi-channel HEMT of the present exampleprovides a large linear region since the ON-state resistance in each ofthe multiple δ-doped layers is low and therefore the ratios among theamounts of current flowing in the respective channel regions can be heldat a nearly constant value. Thus, a HEMT with reduced distortion and ahigh power can be implemented.

In the structure of the HEMT shown in FIG. 19, the impurityconcentration in each of the five n-type doped layers of each of themultiple δ-doped InAlAs layers can be adjusted properly depending on afrequency region in which it is used, an amplification factor, and thelike such that, e.g., a profile in which the concentration graduallydecreases exponentially in the upward direction is provided.

Although the four channel regions are provided in the HEMT of the secondexample shown in FIG. 19, it is also possible to implement a structurein which two or three channel regions or five or more channel regionsare provided. If the multiple δ-doped InAlAs layer 204″ and the multipleδ-doped InGaAs layer 203′ are removed from the HEMT of the presentexample, a structure having only two channel regions, which are thefirst and second channel regions Rch1 and Rch2 formed in the InGaAslayer 203, is obtainable. If the multiple δ-doped InAlAs layer 204″ isremoved from the HEMT of the present example, a structure having onlythree channel regions, which are the first and second channel regionsRch1 and Rch2 formed in the InGaAs layer 203 and the third channelregion Rch3 formed in the InGaAs layer 203′, is obtainable. If an InGaAslayer having substantially the same structure as each of the InGaAslayers 203 and 203′ is provided below the multiple δ-doped InAlAs layer204″ of the HEMT according to the present example, five channel regionsare formed. If an InGaAs layer having substantially the same structureas each of the InGaAs layers 203 and 203′ and a multiple δ-doped InAlAslayer having basically the same structure as each of the multipleδ-doped InAlAs layers 204, 204′, and 204″ are provided below themultiple δ-doped InAlAs layer 204″ of the HEMT according to the presentexample, sixth channel regions are formed. By alternately increasing thenumber of such InGaAs layers or the number of such multiple δ-dopedInAlAs layers by one, therefore, the number of channel regions isincreased by one.

Structures of Components of Communication System

FIG. 21 schematically shows an example of the radio-wave terminal(mobile station) 102 in the communication system shown in FIG. 2. A PDCsystem is adopted here. The RF wireless unit shown in FIG. 21 includes areceived-signal amplifier 122 and the amplified-signal transmitter 123shown in FIG. 2. The controller of the radio-wave terminal 102 as themobile station shown in FIG. 2 is composed of the CPU, the cipherTDMA-CCT, the SP-CODEC, the ROM/RAM, the TERM-ADP, the DPSK-MOD, theHiSpeedSYNTH, the IF-IC, and the CPSK-DEMOD (EQL) shown in FIG. 21.

The linear PA (power amplifier) in the RF wireless unit shown in FIG. 21can be composed of, e.g., the circuit having the HEMT shown in FIG. 19disposed therein. In that case, the HEMT in each of circuits for thecontroller can be composed of the HEMT shown in FIG. 19

FIG. 22 is an electric circuit diagram showing an exemplary circuitstructure of the mixer 134 shown in FIG. 3 or the mixer shown in FIG.21. The example shown here has a local amplifier Specifically, a HEMT 1for amplifying a local signal which receives a local signal Sl₀ at thegate and outputs a signal Sout1 obtained by amplifying the local signalSl₀ and a HEMT 2 for amplifying a mixer signal which receives twosignals Smix1 and Smix2 at the gate and outputs a signal Sout2 obtainedby mixing and amplifying the signals Smix1 and Smix2 are disposed. It ispossible to form the HEMT, the diode, and the capacitor in the circuiton a single InP substrate as shown in, e.g., FIG. 7 and thereby composea single MMIC. Since a resistor element can be regarded as a part of theconductor film of the inductor, the resistor element can be formedextremely easily on the InP substrate, though it is not depicted in FIG.7.

FIG. 23 is an electric circuit diagram showing an example of ahigh-output switch circuit containing a SPDT switch shown in FIG. 21 ora high-output switch circuit disposed in the switch shown in FIG. 3. Inthis example, the high-output switch circuit is so configured as toreceive input signals Sin1 and Sin2 and output a signal Sout obtained byamplifying either of the input signals Sin1 and Sin2. It is possible toform output signal HEMTs 1 to 4, capacitors C1 to C6, diodes D1 and D2,and resistor elements R1 to R6 on a single SiC substrate and therebycompose an MMIC.

Variations

FIG. 24 shows another exemplary structure (first variation) of the mainamplifier shown in FIG. 4 according to the foregoing embodiment. Thefirst variation comprises a previous-stage HEMT and a subsequent-stageHEMT which are amplifying transistors in two stages. On the input sideof the previous-stage HEMT, there is provided an input-side impedanceadjusting circuit having a capacitor C1, a resistor element R1, and aninductor II. Between the previous-stage HEMT and the subsequent-stageHEMT, there is provided an intermediate impedance adjusting circuithaving capacitors C2 and C3, a resistor element R2, and an inductor 12.On the output side of the subsequent-stage MESFET, there is provided anoutput-side impedance adjusting circuit containing a capacitor C4 and aninductor 13.

Each of the elements in the first variation can be composed of a HEMT40, a capacitor 50, and an inductor 60 as shown in FIG. 7. Accordingly,an MMIC composed of the circuit shown in FIG. 24 which is provided on asingle InP substrate can be obtained.

FIG. 25 shows still another exemplary structure (second variation) ofthe main amplifier shown in FIG. 4 according to the foregoingembodiment. The second variation has a structure in which four HEMTs Ato D composing a differential amplifier are disposed in parallel. On theinput side of each of the HEMTs A to D, there are provided an input-sideprematching having a capacitor, a resistor element (not shown), and thelike. On the output side of each of the HEMTs A to D, there are providedan output-side prematching having a capacitor, a resistor (not shown),and the like.

Each of the elements in the second variation can be composed of a HEMT40, a capacitor 50, and an inductor 60 as shown in FIG. 7. Accordingly,an MMIC composed of the circuit shown in FIG. 25 which is provided on asingle InP substrate can be obtained.

FIG. 26 is a block circuit diagram schematically showing the structureof the base station 101 a in the third variation in which the two mainamplifiers 138 are disposed in parallel. In this case also, the two mainamplifiers can be composed of the circuit shown in FIG. 4.

If FIG. 4, FIG. 25, and FIG. 26 are compared, the amplifier circuitsshown in FIG. 25 and FIG. 26 are preferably provided to achieve ahighest amplification factor. However, the structure of the impedancematching circuit is more complicated as a larger number of HEMTs areprovided. In the case of handling a signal in the RF range, particularlya signal on the GHz order, the process for achieving impedance matching(such as trimming) is complicated as a larger number of HEMTs areprovided. Accordingly, the structure of the base station can bedetermined selectively depending on the use and scale thereof.

Other Embodiments

Although the foregoing embodiment has described the example in which theequipment for a communication system according to the present inventionis applied to the base station, the terminal (mobile station), the homeelectrical appliance, and the like in a millimeter-wave communicationsystem, the present invention is not limited to such an embodiment.Examples of the communication system include a mobile phone system, acar phone system, a PHS, and a PDA. By providing the equipment disposedin such a system with the HEMT, the diode, the MESFET, the capacitor,the inductor, and the like shown in FIG. 7, the same effects as achievedby the foregoing embodiment are achievable.

A large-current characteristic and a high breakdown voltage areattainable even if the HEMT shown in FIG. 17 is provided by using asemi-insulating substrate other than the InP substrate, e.g., a GaAssubstrate, a GaN substrate, a Si substrate (a Si/SiGe (or SiGeC) heterostructure), or the like.

Each of the first and second active regions according to the presentinvention can be composed of any one material selected from the groupconsisting of InP, InGaAs, InAlAs, GaN, InGaP, and InGaSb. By providingan InGaSb layer in place of the InGaAs layer (17, 203, or 203′), inparticular, a larger amount ΔVg of band offset between the InGaSb layerand the multiple δ-doped InAlAs layer (13, 204, 204′, or 204″) than inthe foregoing embodiment is ensured so that the efficiency with whichcarriers are confined is further increased and a current driving abilityis further increased.

If the multiple δ-doped layer shown in FIG. 7 composed of the n-typedoped layers (heavily doped layers) and the undoped layers (lightlydoped layers) which are alternately stacked is provided in a deviceother then HEMT such as a heterojunction bipolar transistor, asemiconductor layer, or the like, a large current characteristic and ahigh breakdown voltage characteristic are achievable. In theheterojunction bipolar transistor, a base layer is normally composed ofa material having a band gap smaller than those of an emitter layer anda collector layer. If the emitter layer or the collector layer in thestructure is composed of the multiple δ-doped layer, a bipolartransistor with a high breakdown voltage can be obtained.

Alternatively, if a gate insulating film and a gate electrode arestacked successively in layers on the InP layer 18 of FIG. 7, e.g., aMISFET comprising a multiple δ-doped layer can be obtained, similarly tothe SiC-MESFET disclosed in a PCT application (PCT/JP00/08156). TheMISFET can achieve a high channel mobility and a high breakdown voltage,similarly to the SiC-MISFET disclosed in the foregoing PCT application.By the effects of the increased channel mobility and the increasedbreakdown voltage, a low ON-state resistance with a high breakdownvoltage, a large current capacitance, and a high transconductance areachieved so that a MISFET having the advantages of low power consumptionand a high gain is formed successfully. An RF characteristic isnaturally improved by the increased channel mobility.

Although the foregoing embodiment has used the same material(In_(0.52)Al_(0.48)As or In_(0.53)Ga_(0.47)As) to compose the δ-dopedlayers (heavily doped layers) and the undoped layers (lightly dopedlayers) in the multiple δ-doped layer, a hetero junction portion mayalso be formed therebetween by using different materials to compose theδ-doped layers and the undoped layers.

Even if the InAlAs layer or the InGaAs layer is used, the compositionratio therebetween need not be In_(0.52)Al_(0.48)As orIn_(0.53)Ga_(0.47)As.

INDUSTRIAL APPLICABILITY

A semiconductor device according to the present invention is used for adevice such as a Schottky diode, MESFET, FEMT, or the like mounted onelectric equipment, particularly a device handling an RF signal, a powerdevice, or equipment for a communication system.

1. A semiconductor device comprising: at least one active region stackedunit comprising a first active region disposed on a substrate and asecond active region disposed on the first active region, at least aportion of the first active region functioning as a channel region, andthe second active region including a material having a band gapdifferent from a band gap of the first active region such that a banddiscontinuity occurs between the first and second active regions; a gateelectrode disposed above the second active region located at theuppermost of said at least one active region stacked unit; and a pair ofdiffusion regions of high concentration impurities provided above thesecond active region located at the uppermost of said at least oneactive region stacked unit, and opposing each other with the gateelectrode being interposed therebetween, wherein the second activeregion includes multiple δ-doped layers made by alternately stacking aplurality of first semiconductor layers and a plurality of secondsemiconductor layers, each of the plurality of first semiconductorlayers allows passage of carriers therethrough, each of the plurality ofsecond semiconductor layers contains an impurity for carriers havingconcentration higher than that of each of the plurality of firstsemiconductor layers, and is smaller in film thickness than each of saidplurality of first semiconductor layers, the plurality of firstsemiconductor layers and the plurality of second semiconductor layershave a same composition, and no channel region is provided above thesecond active region located at the uppermost of said at least oneactive region stacked unit.
 2. The semiconductor device of claim 1,wherein the substrate is composed of InP and each of the first andsecond active regions is composed of any one material selected from thegroup consisting of InP, InGaAs, InAlAs, GaN, InGaP, and InGaSb.
 3. Thesemiconductor device of claim 1, which functions as a HEMT, wherein: thesecond semiconductor is composed of a material having a band gap largerthan a band gap of a material composing the first semiconductor, aportion of the first active region adjacent an interface between thefirst and second active regions serves as a channel layer, and thesecond active region functions as a carrier supplying layer. 4.Equipment for a communication system handling an RF signal, theequipment being disposed in the communication system and having anactive element formed by using a semiconductor, the active elementcomprising: at least one active region stacked unite comprising a firstactive region and a second active region disposed on the first activeregion, at least a portion of the first active region functioning as achannel region, and the second active region includes a material havinga band gap different from a band gap of the first active region suchthat a band discontinuity occurs between the first and second activeregions; a gate electrode provided above the second active regionlocated at the uppermost of said at least one active region stackedunit; and a pair of diffusion regions of high concentration impuritiesprovided above the second active region located at the uppermost of saidat least one active region stacked unit, and opposing each other withthe gate electrode being interposed therebetween, wherein the secondactive region comprises multiple δ-doped layers made by alternatelystacking a plurality of first semiconductor layers and a plurality ofsecond semiconductor layers; each of the plurality of firstsemiconductor layers allows passage of carriers therethrough and each ofthe plurality of second semiconductor layers contains an impurity forcarriers having concentration higher than that of each of the pluralityof first semiconductor layers, and is smaller in film thickness thaneach of said plurality of first semiconductor layers, the plurality offirst semiconductor layers and the plurality of second semiconductorlayers have a same composition, and no channel region is provided abovethe second active region located at the uppermost of said at least oneactive region stacked unit.
 5. The equipment for a communication systemof claim 4, wherein a plurality of active region stacked bodies arestacked above the substrate.
 6. The equipment for a communication systemof claim 4, wherein the substrate is composed of InP and each of thefirst and second active regions is composed of any one material selectedfrom the group consisting of InP, InGaAs, InAlAs, GaN, AlGaN, InGaP, andInGaSb.
 7. The equipment for a communication system of claim 4, whereina portion of the first active region of the active element adjacent aninterface between the first and second active regions serves as achannel layer, the second active region functions as a carrier supplyinglayer, and the active clement functions as a HEMT.
 8. The equipment fora communication system of claim 5, further comprising another secondactive region provided directly below the first active region located atthe bottom of said at least one active region stacked unit, wherein theanother second active region comprises multiple δ-doped layers made byalternately stacking the plurality of first semiconductor layers and theplurality of second semiconductor layers, each of the plurality of firstsemiconductors layers allows passage of carriers therethrough, each ofthe plurality of second semiconductor layers contains an impurity forcarriers having concentration higher than that of each of the pluralityof first semiconductor layers, and is smaller in film thickness thaneach of said plurality of first semiconductor layers, and the pluralityof first semiconductor layers and the plurality of second semiconductorlayers have a same composition.
 9. The equipment for a communicationsystem of claim 4, wherein the active element is disposed in atransmitter.
 10. The equipment for a communication system of claim 4,wherein the active element is disposed in a receiver.
 11. The equipmentfor a communication system of claim 4, wherein the active element isdisposed in a mobile data terminal.
 12. The equipment for acommunication system of claim 4, wherein the active element is disposedin a base station.
 13. The equipment for a communication system of claim4, which is a transmitting or receiving module constructed removablyfrom an object under control.
 14. The semiconductor device of claim 1,wherein a plurality of active region stacked bodies are stacked abovethe substrate.
 15. The semiconductor device of claim 14, furthercomprising another second active region provided directly below thefirst active region located at the bottom of said at least one activeregion stacked unit, wherein the another second active region comprisesmultiple δ-doped layers made by alternately stacking the plurality offirst semiconductor layers and the plurality of second semiconductorlayers, each of the plurality of first semiconductor layers allowspassage of carriers therethrough, each of the plurality of secondsemiconductor layers contains an impurity for carriers havingconcentration higher than that of each of the plurality of firstsemiconductor layers, and is smaller in film thickness than each of saidplurality of first semiconductor layers, and the plurality of firstsemiconductor layers and the plurality of second semiconductor layershave a same composition.